util/c11: Update function u_thread_create to be c11 conformance

util/c11: Update function u_thread_create to be c11 conformance

Do not assume thrd_t to be a pointer or integer, as the C11 standard tells us:
  thrd_t: implementation-defined complete object type identifying a thread
At https://en.cppreference.com/w/c/thread
So we always return the thread creation return code instead of thrd_t value, and judge the return
code properly.

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15087>