lib: c: core: implement strchr and strcspn

lib/libc/core/string/strchr.c

@@ -0,0 +1,16 @@
+#include <stddef.h>
+
+char *strchr(const char *str, int c)
+{
+	while (1) {
+		if (*str == c) {
+			return (char *)str;
+		}
+
+		if (*str == 0) {
+			break;
+		}
+	}
+
+	return NULL;
+}