Add a memcpy utility to libc

lib/libc/src/string.cpp

@@ -0,0 +1,10 @@
+#include "string.h"
+
+void* memcpy(void* dest, const void* src, size_t count) {
+  uint8_t* d = (uint8_t*)dest;
+  const uint8_t* s = (uint8_t*)src;
+  for (size_t i = 0; i < count; i++) {
+    d[i] = s[i];
+  }
+  return dest;
+}