Improve regalloc register priority order under Fastcall ABI

On x86-64 with the new backend, the fastcall ABI implementation (#2678) currently results in suboptimal code in many cases because the register allocator's priority order, which is tuned to use SysV caller-saves (volatiles) first, allocates registers that are actually callee-saves and forces unnecessary clobber-saves. We should tune the ordering based on ABI in order to improve the generated code.