Share /machine: handling code with llvm-cvtres too

r363016 let lld-link and llvm-lib share the /machine: parsing code.
This lets llvm-cvtres share it as well.

Making llvm-cvtres depend on llvm-lib seemed a bit strange (it doesn't
need llvm-lib's dependencies on BinaryFormat and BitReader) and I
couldn't find a good place to put this code. Since it's just a few
lines, put it in lib/Object for now.

Differential Revision: https://reviews.llvm.org/D63120

llvm-svn: 363144