Reduce my idiocy in register saving code.

(1) Rename ABI_ALL_CALLEE_SAVED to ABI_ALL_CALLER_SAVED, because that's
what it was actually defined as (and used as).  Derp.

(2) RegistersInUse is always used for the purpose of saving registers
before calling a C++ function in the middle of a JIT block (without
flushing).  There is no need to save callee-saved registers in this
case.  Change the name to CallerSavedRegistersInUse and mask with
ABI_ALL_CALLER_SAVED.

Nothing obvious broke when starting up a Melee game.  (I added a test
for anything actually being masked out; it happens, but in this
particular case seemed to occur at most a few dozen times per second, so
the actual performance benefit is probably negligible.)