You can use preprocessor directive :

#ifdef 65C816
	ldy #512+256
loop2
	lda charset,y
	sta $b500,y
	dey
	bne loop2	
#else
	ldy #0	
loop
	lda charset,y
	sta $b500,y
	lda charset+256,y
	sta $b500+256,y
	lda charset+512,y
	sta $b500+512,y
	iny
	bne loop
#endif
	rts