[WebAssembly] Factor commonality between wasm32 and wasm64 in test/Preprocessor/init.c

Use the -check-prefixes= feature to merge most of the WEBASSEMBLY32 and
WEBASSEMBLY64 test checks into a shared WEBASSEMBLY test check.

Differential Revision: https://reviews.llvm.org/D57153

llvm-svn: 352099