[libcxx] Direct support for Fuchsia

Fuchsia's libc was forked from musl, but has evolved sufficiently
since then so it no longer makes sense to pretend it's musl. This
change implements direct support for Fuchsia rather than
piggybacking on musl support.

Differential Revision: https://reviews.llvm.org/D31970

llvm-svn: 300261