[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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@351163 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed