[clang][Emscripten] Define __unix family of macros

This will allow us to remove these from the downstream
driver:
https://github.com/emscripten-core/emscripten/blob/57270ce8150a5107e591b4e9ec7cbeff0ba7c905/emcc.py#L860-L863

Differential Revision: https://reviews.llvm.org/D108735