Symbolicize explicit uses of x86 registers where possible (GPRs only for now).

Uses are split into three categories:

- Arbitrary (except for size savings) - constants like RSCRATCH are
used.

- ABI (i.e. RAX as return value) - ABI_RETURN is used.

- Fixed by architecture (RCX shifts, RDX/RAX for some instructions) -
explicit register is kept.

In theory this allows the assignments to be modified easily.  I verified
that I was able to run Melee with all the registers changed, although
there may be issues if RSCRATCH[2] and ABI_PARAM{1,2} conflict.