[libc] optimize _isascii

Author: mateoconlechuga

src/libc/isascii.src

@@ -4,10 +4,8 @@
 	public	_isascii
 _isascii:
 	pop	de
-	pop	hl
-	push	hl
+	ex	(sp),hl
 	push	de
-
 	ld	de,-128
 	add	hl,de
 	sbc	hl,hl