Missing RTS flag for the moving/copying GC

Motivation

We have an application that appears to benefit from the new nonmoving GC. We want to make it default for users (by baking in -with-rtsopts), but also allow users to override this choice if it turns out to be bad for them. There doesn't appear to be a way to do that.

Proposal

Add a new --copying-gc or --moving-gc flag, that can override the baked in choice.

...or maybe this is a documentation issue.