cgen: add fixed array bounds checking for non-literal index (#8832)

vlib/builtin/builtin.c.v

@@ -274,3 +274,14 @@ pub fn is_atty(fd int) int {
 		return C.isatty(fd)
 	}
 }
+
+[inline]
+fn v_fixed_index(i int, len int) int {
+	$if !no_bounds_checking ? {
+		if i < 0 || i >= len {
+			s := 'fixed array index out of range (index: $i, len: $len)'
+			panic(s)
+		}
+	}
+	return i
+}