[WebAssembly] Test the "wasm32-wasi" triple

Add triple tests for "wasm32-wasi" and "wasm64-wasi", and also remove the
"-musl" component from the existing wasm triple tests as we're not using that
in practice (WASI libc is derived in part from musl, but it is not fully
musl-compatible).

Differential Revision: https://reviews.llvm.org/D61334

Reviewer: sbc100
llvm-svn: 359629