Check the `-mlong-calls` command line option and pass the `long-calls`
feature flag to the backend. Handling of this feature flag in the backend
needs to be implemented by a separate commit.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@307386 91177308-0d34-0410-b5e6-96231b3b80d8