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 (except for the MTX_INIT and CND_INIT macros which
are Fuchsia extensions), and could be used by other platforms that use
C11 threads rather than pthreads in the future.