This allows us to save and restore callee-save temporaries, i.e. RBP on
32-bit x86. Otherwise it's a disaster shuffling stack arguments using
temporaries.
The x32 ABI (i.e., amd64 with 32-bit pointers) is mostly a C language
question. From a JIT perspective it mostly looks just like amd64. We
can fix any differences in a followup, but x32 is also a pretty dead
ABI, so I feel OK with this.
Now that there's no hazard to using a register used for passing
arguments, renumber to give JIT_R/JIT_F/JIT_V names to all registers.
Choose a temp register that's not used for passing arguments. Our
previous choice was an argument register (doh!) which made function
calls with many arguments fail.
This improves integration with other projects. Like for example Guile
already has files named jit.c and jit.h; it's easier to manage if
lightening uses its own file names.
