Optimized __setflag

[ZERICO2005]

The compiler will emit call pe, __setflag after signed comparisons. I have added our own faster implementation of __setflag to the CRT.

__setflag:
	ret	po
; __invertflag:
	push	af
	ex	(sp), hl
	ld	a, l
	xor	a, $80	; invert S flag
	ld	l, a
	ex	(sp), hl
	pop	af
	ret

Since the compiler only appears to emit call pe, __setflag instead of call __setflag, it might be better to have it call __invertflag so it can skip ret po

[mateoconlechuga]

This seems sort of risky to me