Document cross-compilation process

[xrme]

Documentation on cross-compilation is scattered to non-existent.

There are two cases of interest, I think.

The first case is using, say, an x86-64 host to compile for an ARM target. I've done this before, and it's not too bad.

The second case would be using, say, an x86-64 host to compile for an x86-64 target that needs to change in some incompatible way. For example, we might need to change where in low memory NIL lives, or might need to change register usage in some way. I've never done this, and I don't know if it's even possible.