[WebAssembly] Parsing missing directives to produce valid .o

Summary:
The assembler was able to assemble and then dump back to .s, but
was failing to parse certain directives necessary for valid .o
output:
- .type directives are now recognized to distinguish function symbols
  and others.
- .size is now parsed to provide function size.
- .globaltype (introduced in https://reviews.llvm.org/D54012) is now
  recognized to ensure symbols like __stack_pointer have a proper type
  set for both .s and .o output.

Also added tests for the above.

Reviewers: sbc100, dschuff

Subscribers: jgravelle-google, aheejin, dexonsmith, kristina, llvm-commits, sunfish

Differential Revision: https://reviews.llvm.org/D53842

llvm-svn: 346047