[libcxx] Temporarily switch back to pthread backend for Fuchsia

We switched to C11 thread API on Fuchsia in ab9aefee, but further
testing showed that Fuchsia's C11 mutex implementation needs a few
improvements for this to be usable, so we temporarily switch back
to the pthread implementation until those issues are addressed.

Differential Revision: https://reviews.llvm.org/D72862