[clang][WebAssembly] Only expose wait and notify builtins with atomics

Summary:
Since the underlying wait and notify instructions are only available
when the atomics feature is enabled, it only makes sense to expose
their builtin functions when atomics are enabled.

Reviewers: aheejin, sunfish

Subscribers: dschuff, sbc100, jgravelle-google, jfb, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79534