Rather than keep a map that will have to be constantly updated every time
a new argument gets added to a compiler, the 'additionalArguments' option
will allow users to directly add arguments to the underlying compiler.
Provides a few benefits:
1) The new package has the correct dependencies listed, so I removed the
-MissingDependenciesOK flag from the crossgen scripts.
2) The new compiler supports -publicSign (formerly called "OSS sign"),
which I patched through dotnet-compile-csc .
