Simplified WebAssemblyAsmBackend by removing explicit ELF variant.

The ELF version was broken (does not deal with wasm specific fixups),
and now is slightly less broken. It will be removed in its entirety
in the future which this change makes slightly easier (just remove
the IsELF bool).

Differential Revision: https://reviews.llvm.org/D47745

Patch by Wouter van Oortmerssen

llvm-svn: 333964