Handle floating register types in mem2reg/reg2mem
x86_overlay_regpair handles floats
Hook func_ret_regs into the x86_overlay_pair logic
Rename strets -> float/int register strets
Remove struct_call, rename ABI makefile
Add ifdef IMPL checks to float/int stret example
%.0f for int/float stret printfs
Specify C99 in ABI.mk so we get things like main/return 0