[libcxx] Use C11 thread API on Fuchsia

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.

Differential Revision: https://reviews.llvm.org/D64378