* It will make snapshot more precise

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.

Makefile: fix Makefile for noop difftest

See merge request projectn/nemu!64

Sdi difftest

See merge request projectn/nemu!62

* riscv64 does not work. We will fix it after merging to the
  `sdi-spill` branch

* SDI can not distinguish I/O accessing. Therefore, REF may execute
  instructions accessing devices. We should add dummy address spaces to
  receive the write. But reads from device are not supported.
* For SDI, now the backend will map 0xa10003f8 to two different I/O
  spaces. Only the first one will be returned by find_mapid_by_addr().
  So we should also call backend_init_device() before backend_init() to
  let find_mapid_by_addr() return the real serial instead of the dummy
  serial.

* see commit 4660a8af

* Makefiles in nexus-am may first define ENGINE

Nemu ref

See merge request projectn/nemu!61

* we may enable CSR checking in the future

Word rtl

See merge request projectn/nemu!60

* this can save an rtl_sext() in exec_lds() for riscv64/riscv32/mips32