Move the platform dependent functions from u_thread.h to u_thread.c,
so that avoid pull unneeded headers when using u_thread.h, including the big windows.h
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/18425>
