Add implementations of memcmp and memcpy to ulib

This is necessary because gcc may generate calls to memcmp, memset,
memcpy, and memmove when compiling with -nostdlib.

user/ulib.c

@@ -107,3 +107,23 @@ memmove(void *vdst, const void *vsrc, int n)
     *dst++ = *src++;
   return vdst;
 }
+
+int
+memcmp(const void *s1, const void *s2, uint n)
+{
+  const char *p1 = s1, *p2 = s2;
+  while (n-- > 0) {
+    if (*p1 != *p2) {
+      return *p1 - *p2;
+    }
+    p1++;
+    p2++;
+  }
+  return 0;
+}
+
+void *
+memcpy(void *dst, const void *src, uint n)
+{
+  return memmove(dst, src, n);
+}

user/user.h

@@ -38,3 +38,5 @@ void* memset(void*, int, uint);
 void* malloc(uint);
 void free(void*);
 int atoi(const char*);
+int memcmp(const void *, const void *, uint);
+void *memcpy(void *, const void *, uint);