add command line option to test compilation for other OS

This helped me setting the needed nothrow annotations for https://github.com/D-Programming-Language/druntime/pull/752 without having to wait for the autotester for OSs that I don't have access to. Maybe this is useful for others, too.

The command line argument "-target=<os>" sets predefined versions according to the given OS. Code generation and object file format are still the same as the OS dmd is built on, so it is no cross compilation. Because of this I'd rather leave this as an undocumented feature for dmd developers.

BTW: druntime did not compile for OpenBSD, is this platform not supported anymore or just not tested by the autotester?