Fix more namespace issues in sys/ucontext.h (bug 21457).

Continuing the fixes for namespace issues in sys/ucontext.h, this
patch moves various symbols into the implementation namespace in the
absence of __USE_MISC.  As with previous changes, it is nonexhaustive,
just covering more straightforward cases.

Structure fields are generally changed to have a prefix __ in the
absence of __USE_MISC, via a macro __ctx (used without a space before
the open parenthesis, since the result is a single identifier).
Various macros such as NGREG also have leading __ added.  No changes
are made to structure tags (and thus to C++ name mangling), except
that in the (unused) file sysdeps/i386/sys/ucontext.h, structures
defined inside other structures as the type for a field have their
tags removed in the non-__USE_MISC case (those structure tags would
not in any case have been visible in C++, because in C++ the scope of
such a tag is limited to the containing structure).  No changes are
made to the contents of bits/sigcontext.h, or to whether it is
included.  Because of remaining namespace issues, this patch does not
yet fix the bug or allow any XFAILs to be removed.

Tested for x86_64 and x86, and with build-many-glibcs.py.

	[BZ #21457]
	* sysdeps/arm/sys/ucontext.h (NGREG): Rename to __NGREG and define
	NGREG to __NGREG if [__USE_MISC].
	(gregset_t): Define using __NGREG.
	(__ctx): New macro.
	(mcontext_t): Use __ctx in defining fields.
	* sysdeps/i386/sys/ucontext.h (NGREG): Rename to __NGREG and
	define NGREG to __NGREG if [__USE_MISC].
	(gregset_t): Define using __NGREG.
	(__ctx): New macro.
	(__ctxt): Likewise.
	(fpregset_t): Use __ctx and __ctxt in defining fields.
	(mcontext_t): Likewise.
	* sysdeps/m68k/sys/ucontext.h (NGREG): Rename to __NGREG and
	define NGREG to __NGREG if [__USE_MISC].
	(gregset_t): Define using __NGREG.
	(__ctx): New macro.
	(mcontext_t): Use __ctx in defining fields.
	* sysdeps/mips/sys/ucontext.h (NGREG): Rename to __NGREG and
	define NGREG to __NGREG if [__USE_MISC].
	(gregset_t): Define using __NGREG.
	(__ctx): New macro.
	(fpregset_t): Use __ctx in defining fields.
	(mcontext_t): Likewise.
	* sysdeps/unix/sysv/linux/alpha/sys/ucontext.h (NGREG): Rename to
	__NGREG and define NGREG to __NGREG if [__USE_MISC].
	(gregset_t): Define using __NGREG.
	(NFPREG): Rename to __NFPREG and define NFPREG to __NFPREG if
	[__USE_MISC].
	(fpregset_t): Define using __NFPREG.
	* sysdeps/unix/sysv/linux/m68k/sys/ucontext.h (NGREG): Rename to
	__NGREG and define NGREG to __NGREG if [__USE_MISC].
	(gregset_t): Define using __NGREG.
	(__ctx): New macro.
	(fpregset_t): Use __ctx in defining fields.
	(mcontext_t): Likewise.
	* sysdeps/unix/sysv/linux/mips/sys/ucontext.h (NGREG): Rename to
	__NGREG and define NGREG to __NGREG if [__USE_MISC].
	(NFPREG): Rename to __NFPREG and define NFPREG to __NFPREG if
	[__USE_MISC].
	(gregset_t): Define using __NGREG.
	(__ctx): New macro.
	(fpregset_t): Use __ctx in defining fields.
	(mcontext_t): Likewise.
	* sysdeps/unix/sysv/linux/nios2/sys/ucontext.h (__ctx): New macro.
	(mcontext_t): Use __ctx in defining fields.
	* sysdeps/unix/sysv/linux/powerpc/sys/ucontext.h (__ctx): New
	macro.
	[__WORDSIZE == 32] (NGREG): Rename to __NGREG and define NGREG to
	__NGREG if [__USE_MISC].
	[__WORDSIZE == 32] (gregset_t): Define using __NGREG.
	[__WORDSIZE == 32] (fpregset_t): Use __ctx in defining fields.
	(mcontext_t): Likewise.
	[__WORDSIZE != 32] (NGREG): Rename to __NGREG and define NGREG to
	__NGREG if [__USE_MISC].
	[__WORDSIZE != 32] (NFPREG): Rename to __NFPREG and define NFPREG
	to __NFPREG if [__USE_MISC].
	[__WORDSIZE != 32] (NVRREG): Rename to __NVRREG and define NVRREG
	to __NVRREG if [__USE_MISC].
	[__WORDSIZE != 32] (gregset_t): Define using __NGREG.
	[__WORDSIZE != 32] (fpregset_t): Define using __NFPREG.
	[__WORDSIZE != 32] (vscr_t): Use __ctx in defining fields.
	[__WORDSIZE != 32] (vrregset_t): Likewise.
	[__WORDSIZE != 32] (mcontext_t): Likewise.
	* sysdeps/unix/sysv/linux/s390/sys/ucontext.h (__ctx): New macro.
	(__psw_t): Use __ctx in defining fields.
	(NGREG): Rename to __NGREG and define NGREG to __NGREG if
	[__USE_MISC].
	(gregset_t): Define using __NGREG.
	(fpreg_t): Use __ctx in defining fields.
	(fpregset_t): Likewise.
	(mcontext_t): Likewise.
	* sysdeps/unix/sysv/linux/sh/sys/ucontext.h (NGREG): Rename to
	__NGREG and define NGREG to __NGREG if [__USE_MISC].
	(gregset_t): Define using __NGREG.
	(NFPREG): Rename to __NFPREG and define NFPREG to __NFPREG if
	[__USE_MISC].
	(fpregset_t): Define using __NFPREG.
	(__ctx): New macro.
	(mcontext_t): Use __ctx in defining fields.
	* sysdeps/unix/sysv/linux/x86/sys/ucontext.h (__ctx): New macro.
	[__x86_64__] (NGREG): Rename to __NGREG and define NGREG to
	__NGREG if [__USE_MISC].
	[__x86_64__] (gregset_t): Define using __NGREG.
	[__x86_64__] (struct _libc_fpxreg): Use __ctx in defining fields.
	[__x86_64__] (struct _libc_fpstate): Likewise.
	[__x86_64__] (mcontext_t): Likewise.
	[!__x86_64__] (NGREG): Rename to __NGREG and define NGREG to
	__NGREG if [__USE_MISC].
	[!__x86_64__] (gregset_t): Define using __NGREG.
	[!__x86_64__] (struct _libc_fpreg): Use __ctx in defining fields.
	[!__x86_64__] (struct _libc_fpstate): Likewise.
	[!__x86_64__] (mcontext_t): Likewise.

sysdeps/unix/sysv/linux/s390/sys/ucontext.h

@@ -26,11 +26,17 @@
 #include <bits/types/stack_t.h>
 
 
+#ifdef __USE_MISC
+# define __ctx(fld) fld
+#else
+# define __ctx(fld) __ ## fld
+#endif
+
 /* Type for a program status word.  */
 typedef struct
 {
-  unsigned long mask;
-  unsigned long addr;
+  unsigned long __ctx(mask);
+  unsigned long __ctx(addr);
 } __attribute__ ((__aligned__(8))) __psw_t;
 
 /* Type for a general-purpose register.  */
@@ -42,35 +48,40 @@ typedef unsigned long greg_t;
    that has the same size as s390_regs.  This is needed for the
    elf_prstatus structure.  */
 #if __WORDSIZE == 64
-# define NGREG 27
+# define __NGREG 27
 #else
-# define NGREG 36
+# define __NGREG 36
+#endif
+#ifdef __USE_MISC
+# define NGREG __NGREG
 #endif
 /* Must match kernels psw_t alignment.  */
-typedef greg_t gregset_t[NGREG] __attribute__ ((__aligned__(8)));
+typedef greg_t gregset_t[__NGREG] __attribute__ ((__aligned__(8)));
 
 typedef union
   {
-    double  d;
-    float   f;
+    double  __ctx(d);
+    float   __ctx(f);
   } fpreg_t;
 
 /* Register set for the floating-point registers.  */
 typedef struct
   {
-    unsigned int fpc;
-    fpreg_t fprs[16];
+    unsigned int __ctx(fpc);
+    fpreg_t __ctx(fprs)[16];
   } fpregset_t;
 
 /* Context to describe whole processor state.  */
 typedef struct
   {
-    __psw_t psw;
-    unsigned long gregs[16];
-    unsigned int aregs[16];
-    fpregset_t fpregs;
+    __psw_t __ctx(psw);
+    unsigned long __ctx(gregs)[16];
+    unsigned int __ctx(aregs)[16];
+    fpregset_t __ctx(fpregs);
   } mcontext_t;
 
+#undef __ctx
+
 /* Userlevel context.  */
 typedef struct ucontext
   {