[WebAssembly] Add mutable-globals to bleeding-edge CPU

Summary: This brings the backend in line with Clang.

Reviewers: aheejin, dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60594

llvm-svn: 358310