monitor,difftest,ref: add difftest_memcpy_from_ref()

* Snapshot in XiangShan needs to save the state of memory of NEMU to
  enable difftest. This is because there is cache inside the RTL design,
  which makes the state of the RTL memory different from the one in
  NEMU.

src/monitor/difftest/ref.c

@@ -9,6 +9,10 @@ void difftest_memcpy_from_dut(paddr_t dest, void *src, size_t n) {
   memcpy(guest_to_host(dest - PMEM_BASE), src, n);
 }
 
+void difftest_memcpy_from_ref(void *dest, paddr_t src, size_t n) {
+  memcpy(dest, guest_to_host(src - PMEM_BASE), n);
+}
+
 void difftest_getregs(void *r) {
   isa_difftest_getregs(r);
 }