[WebAssembly] Add skeleton MC support for the Wasm container format

This just adds the basic skeleton for supporting a new object file format.
All of the actual encoding will be implemented in followup patches.

Differential Revision: https://reviews.llvm.org/D26722


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@295803 91177308-0d34-0410-b5e6-96231b3b80d8
35 files changed