[WebAssembly] Change type of wake count to unsigned int

Summary:
We discussed this at the Nov 12th CG meeting, and decided to use the
unsigned semantics for the wake count.
Corresponding spec change:
https://github.com/WebAssembly/threads/pull/110

Reviewers: sbc100

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

Differential Revision: https://reviews.llvm.org/D54572

llvm-svn: 347005