[WebAssembly] Support multilibs for wasm32 and add a wasm OS that uses it

This adds support for multilib paths for wasm32 targets, following
[Debian's Multiarch conventions], and also adds an experimental OS name in
order to test it. 

[Debian's Multiarch conventions]: https://wiki.debian.org/Multiarch/

Differential Revision: https://reviews.llvm.org/D56553

llvm-svn: 351164