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:
	lea invalid_msg(%rip), %rdi
	mov %ecx, %esi
	xor %rax, %rax
	call printf
	mov $1, %rdi
	call exit