[Bug middle-end/96200] Implement __builtin_thread_pointer() and __builtin_set_thread_pointer() if TLS is supported

[carlos at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96200

--- Comment #2 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Florian Weimer from comment #1)
> __builtin_set_thread_pointer has little value from a glibc perspective
> because when used in application code, it will always result in undefined
> behavior.

Correct, but it's useful for a C library implemetnation. In glibc the aarch64
tls.h is made quite clean by the use of __builtin_thread_pointer and
__builtin_set_thread_pointer.