[WebAssembly] Error when using wasm64 for ISel

Summary:
64-bit WebAssembly (wasm64) is not specified and not supported in the
WebAssembly backend. We do have support for it in clang, however, and
we would like to keep that support because we expect wasm64 to be
specified and supported in the future. For now add an error when
trying to use wasm64 from the backend to minimize user confusion from
unexplained crashes.

Reviewers: aheejin, dschuff, sunfish

Subscribers: sbc100, jgravelle-google, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68254

llvm-svn: 373493