move gcc required functions to string.c

src/lib/string.c

@@ -0,0 +1,10 @@
+#include <stddef.h>
+#include "string.h"
+
+void *memset(void *s, int c, size_t n) {
+    unsigned char *p = s;
+    while (n--) {
+        *p++ = (unsigned char)c;
+    }
+    return s;
+}