Add a way to specify the OS rather than relying upon os.name

In order to do cross-compilations (i.e. compile iOS on MacOSX or Android on Linux), it would be very helpful to be able to specify the OS from command line via a system property (similar to how os.arch can be specified).

This patch provides parameterization for nar.os (since you cannot change os.name without breaking other parts of maven) and also changes the parameterization for architecture to nar.arch (for consistency)