[WebAssembly] Fixed missing "global" symbol type in AsmParser.

Summary:
These are emitted by the wasm backend for e.g.
__stack_pointer@GLOBAL which previously wasn't accepted by the
assembler.

Reviewers: aheejin, dschuff

Subscribers: sbc100, jgravelle-google, llvm-commits, sunfish

Differential Revision: https://reviews.llvm.org/D52911

llvm-svn: 343830