nptl: Fix pthread_getattr_np when modules with execstack are allowed (BZ 32897)

The BZ 32653 fix (12a497c716) kept the
stack pointer zeroing from make_main_stack_executable on
_dl_make_stack_executable.  However, previously the 'stack_endp'
pointed to temporary variable created before the call of
_dl_map_object_from_fd; while now we use the __libc_stack_end
directly.

Since pthread_getattr_np relies on correct __libc_stack_end, if
_dl_make_stack_executable is called (for instance, when
glibc.rtld.execstack=2 is set) __libc_stack_end will be set to zero,
and the call will always fail.

The __libc_stack_end zero was used a mitigation hardening, but since
52a01100ad it is used solely on
pthread_getattr_np code.  So there is no point in zeroing anymore.

Checked on x86_64-linux-gnu and i686-linux-gnu.
Reviewed-by: Sam James <sam@gentoo.org>

sysdeps/unix/sysv/linux/dl-execstack.c

@@ -19,10 +19,10 @@
 #include <ldsodefs.h>
 
 int
-_dl_make_stack_executable (void **stack_endp)
+_dl_make_stack_executable (const void *stack_endp)
 {
   /* This gives us the highest/lowest page that needs to be changed.  */
-  uintptr_t page = ((uintptr_t) *stack_endp
+  uintptr_t page = ((uintptr_t) stack_endp
 		    & -(intptr_t) GLRO(dl_pagesize));
 
   if (__mprotect ((void *) page, GLRO(dl_pagesize),
@@ -35,9 +35,6 @@ _dl_make_stack_executable (void **stack_endp)
 		  ) != 0)
     return errno;
 
-  /* Clear the address.  */
-  *stack_endp = NULL;
-
   /* Remember that we changed the permission.  */
   GL(dl_stack_flags) |= PF_X;