The GetTID() now uses the value of the current thread handle from
zx_thread_self() on Fuchsia for the thread ID. This is better than the default
implementation which uses pthread_self(), since that results in a 64-bit
pointer being truncated into the 32-bit pid_t type.

PiperOrigin-RevId: 660270084
Change-Id: I7c8b4a26cca775be62eb6f6926cb734cbb893585