[WebAssembly] Enable an atomic.notify MC test

Summary:
Assembly of atomic.notify has been fixed in r364576, so we can enable
it.

Reviewers: aardappel

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63898

llvm-svn: 364596