[WebAssembly] Improve support for WebAssembly binary format

Mostly this change adds support converting to and from
YAML which will allow us to write more test cases for
the WebAssembly MC and lld ports.

Better support for objdump, readelf, and nm will be in
followup CLs.

I had to update the two wasm test binaries because they
used the old style 'name' section which is no longer
supported.

Differential Revision: https://reviews.llvm.org/D31099

Patch by Sam Clegg

llvm-svn: 299101