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

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

The BZ 32653 fix (12a497c716f0a06be5946cabb8c3ec22a079771e) 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
52a01100ad011293197637e42b5be1a479a2f4ae 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>