On Fuchsia, pthread API is emulated on top of C11 thread API. Using C11
thread API directly is more efficient.
While this implementation is only used by Fuchsia at the moment, it's
not Fuchsia specific, and could be used by other platforms that use C11
threads rather than pthreads in the future.
Switching the implementation used by Fuchsia is ABI breaking. Are we 100% certain that all of our Fuchsia users don't expect ABI stability?