Update ARM and x86 ArchHandler to match 64bits counterparts. NFC

Summary:
Define an explicit type for arch specific reference kind and use it in switch statement to make the compiler emit warnings if some case is not cover.
It will help to catch such errors when we add new mach-o reference kind.

Reviewers: shankarke, kledzik

Reviewed By: shankarke

Subscribers: shankarke, aemerson, llvm-commits

Projects: #lld

Differential Revision: http://reviews.llvm.org/D7612

llvm-svn: 229246