elf: Move vDSO setup to rtld (BZ#24967)

This patch moves the vDSO setup from libc to loader code, just after
the vDSO link_map setup.  For static case the initialization
is moved to _dl_non_dynamic_init instead.

Instead of using the mangled pointer, the vDSO data is set as
attribute_relro (on _rtld_global_ro for shared or _dl_vdso_* for
static).  It is read-only even with partial relro.

It fixes BZ#24967 now that the vDSO pointer is setup earlier than
malloc interposition is called.

Also, vDSO calls should not be a problem for static dlopen as
indicated by BZ#20802.  The vDSO pointer would be zero-initialized
and the syscall will be issued instead.

Checked on x86_64-linux-gnu, i686-linux-gnu, aarch64-linux-gnu,
arm-linux-gnueabihf, powerpc64le-linux-gnu, powerpc64-linux-gnu,
powerpc-linux-gnu, s390x-linux-gnu, sparc64-linux-gnu, and
sparcv9-linux-gnu.  I also run some tests on mips.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

sysdeps/unix/sysv/linux/sysdep-vdso.h

@@ -19,8 +19,7 @@
 #ifndef SYSDEP_VDSO_LINUX_H
 # define SYSDEP_VDSO_LINUX_H
 
-#include <dl-vdso.h>
-#include <libc-vdso.h>
+#include <ldsodefs.h>
 
 #ifndef INTERNAL_VSYSCALL_CALL
 # define INTERNAL_VSYSCALL_CALL(funcptr, err, nr, args...)		      \
@@ -34,8 +33,7 @@
     INTERNAL_SYSCALL_DECL (sc_err);					      \
     long int sc_ret;							      \
 									      \
-    __typeof (__vdso_##name) vdsop = __vdso_##name;			      \
-    PTR_DEMANGLE (vdsop);						      \
+    __typeof (GLRO(dl_vdso_##name)) vdsop = GLRO(dl_vdso_##name);	      \
     if (vdsop != NULL)							      \
       {									      \
 	sc_ret = INTERNAL_VSYSCALL_CALL (vdsop, sc_err, nr, ##args);	      \