MC: simplify object file selection for Windows

Windows always uses COFF unless Windows ELF is in use.  Rather than checking if
Windows, MinGW, or Cygwin is being targeted, just check if the target OS is
windows and that it is not an ELF environment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203159 91177308-0d34-0410-b5e6-96231b3b80d8