gnu: add sched_getcpu()

on x86_64 implemented with rdtscp or rdpid, generically with a syscall.

Change-Id: I8f776848bf35575abec8a8c612c4a25d8550daea
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6866
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>