[llvm-rc] Support joined or separate spelling for /fo flag

CMake invokes rc using the joined spelling which appears to be supported
by Microsoft's rc implementation, so we should support it as well.

Differential Revision: https://reviews.llvm.org/D54191

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346470 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed