Reapply "MC: simplify object file selection for Windows"

That was overly aggressive in assuming that we could always assume COFF.  Some
of the tests assume that they will get ELF rather than COFF even on Windows
where the default is COFF.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203176 91177308-0d34-0410-b5e6-96231b3b80d8