* they can be easily mapped to riscv instructions

* now the set of API to abstract the behavior of different ISAs are
  listed in include/isa.h
* ISA-dependent structure and macro definitions used by non-ISA code are
  listed in include/$ISA.h

* This can let gdb catch the abort signal to observe detail state of
  NEMU, instead of exiting.

* this makes it easy to filter unnecessary log far from the crash point

* other scripts can report an error when NEMU does not return
  successfully