[libc] add _isblank

Author: mateoconlechuga

src/libc/include/ctype.h

@@ -9,6 +9,8 @@ int isalnum(int c);
 
 int isalpha(int c);
 
+int isblank(int c);
+
 int iscntrl(int c);
 
 int isdigit(int c);

src/libc/isblank.src

@@ -0,0 +1,16 @@
+	assume	adl=1
+
+	section	.text
+	public	_isblank
+_isblank:
+	pop	de
+	ex	(sp),hl
+	push	de
+	ld	a,l
+	cp	a,32
+	ret	z
+	cp	a,9
+	ret	z
+	or	a,a
+	sbc	hl,hl
+	ret