Fix handling of code writes for Z80 core. This seems to get things close to being back to where they were before the big refactor that broke the Z80 core. Some problems remain. Notably the sound driver in Sonic 2 is still quite broken.

invalid_msg:
	.asciz "Invalid instruction at %X\n"

	.global m68k_invalid
m68k_invalid:
	push %ecx
	push invalid_msg
	xor %eax, %eax
	call printf
	push $1
	call exit