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

* this makes the behavior of rtl_lm() more similar to the CPU
  instructions

Makefile: clean kvm

See merge request projectn/nemu!59

Refactor api

See merge request projectn/nemu!58

* exceptions should distinguish between read and ifetch

* we may add another engine to use ROCC to communicate with jmpctl in
  the future

* this yields more chance to inline functions

Riscv zero

See merge request projectn/nemu!57

* the hardware will do this for us

Audio

See merge request projectn/nemu!56

Misc

See merge request projectn/nemu!55

* now all guest of the rv64 engine starts from 0x80000000