struct_call.mk can abi-compile any target file
x86 has three return kinds - stret, scalar reg and struct-regs
Pull out x86 stret memcpy() logic
Added x86 stret via-regs logic
Framework for ret-reg-spilling
Pulled out mem2regs overlay logic
Overlay works both ways - mem2reg and reg2mem
impl_call handles setting up the return stret pointer, etc
Emit comments for __builtin_memcpy() and overlay-reg/mem copies
Updated struct_call.c example