[WebAssembly] Add missing -mattr=+exception-handling guards

Summary:
The use of exception handling instructions should only be enabled with
`-mattr=+exception-handling` option.

Reviewers: jgravelle-google

Subscribers: dschuff, sbc100, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D49391

llvm-svn: 337425