[PECOFF] Run cvtres.exe with /machine:x64.

If we are creating a PE+ executable, we need to run cvtres with
/machine:x64 instead of /machine:x86. Otherwise the resulting executable
would be invalid.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@217214 91177308-0d34-0410-b5e6-96231b3b80d8