[WebAssembly] Don't use default GetLinkerPath

We can't (don't want to) honor the same set of "-fuse-ld" flags with
WebAssembly since the ELF linkers (ld.lld, ld.gnu, etc) don't work with
wasm object files.

Instead we implement our own linker finding logic, similar or other
non-ELF platforms like MSVC.

We've had a few issues with CLANG_DEFAULT_LINKER overriding the
WebAssembly linker which doesn't make sense since there is no generic
linker that can handle WebAssembly today.

Differential Revision: https://reviews.llvm.org/D59743

llvm-svn: 356953