[AArch64-emitter] Initial work on a emitter for 64bit ARM.

This being the initial work on an emitter will be buggy and mostly untested.
I've tested a few instruction encodings and am expecting most to work as long as one stays away from VFP/SIMD.
This implements mostly enough instructions to bring up an initial JIT with integer support.
There also aren't any safeguards in place, so if one passes too large of an IMM or the wrong register it'll break.

I want some initial code reviews on this, not to actually merge it yet.