[WebAsembly] Update default triple in test files to wasm32-unknown-unkown.

Summary: The final -wasm component has been the default for some time now.

Subscribers: jfb, dschuff, jgravelle-google, eraman, aheejin, JDevlieghere, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D46342

llvm-svn: 332007