[WebAssembly] Revert type of wake count in atomic.wake to i32

Summary:
We decided to revert this from i64 to i32 in Nov 28 CG meeting. Fixes
PR38632.

Reviewers: dschuff

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

Differential Revision: https://reviews.llvm.org/D51013

llvm-svn: 340235