Ignore some -f options that are supported by gcc.

It is possible that we should say some of these are unsupported, but this is
not any worse than the old behavior of ignoring all unknown -f options.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@191429 91177308-0d34-0410-b5e6-96231b3b80d8