Revert r304117 - WebAssembly object format isn't ready to be the default

Summary: Wasm object format has some functionality regressions from the ELF format, and doesn't play nicely with the rest of the toolchain. It should eventually be the default, but not yet.

Reviewers: sunfish, sbc100

Subscribers: jfb, dschuff, llvm-commits

Differential Revision: https://reviews.llvm.org/D33811

llvm-svn: 304512