[WebAssembly] WASI support for libcxx
This adds explicit support for the WASI platform to libcxx.
WASI libc uses some components from musl, however it's not fully compatible
with musl, so we're planning to stop using _LIBCPP_HAS_MUSL_LIBC and
customize for WASI libc specifically.
Differential Revision: https://reviews.llvm.org/D61336
Reviewers: sbc100, ldionne