[WebAssembly] Add extra test for weak global symbols

Summary:
Currently the test only checks behaviour for weak function symbols.

Should be good to merge straight away?

Reviewers: sbc100

Reviewed By: sbc100

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

Differential Revision: https://reviews.llvm.org/D41449

llvm-svn: 321308