[WebAssembly] Support for binary atomic RMW instructions

Summary:
This adds support for binary atomic read-modify-write instructions:
add, sub, and, or, xor, and xchg.

This does not yet support translations of some of LLVM IR atomicrmw
instructions (nand, max, min, umax, and umin) that do not have a direct
counterpart in wasm instructions.

Reviewers: dschuff

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

Differential Revision: https://reviews.llvm.org/D49088

llvm-svn: 336615