Improve debug

See merge request projectn/nemu!20

* 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

riscv64,mmu: fix wrong mode for sv39

See merge request projectn/nemu!19

riscv64,mmu: refactor page_walk

See merge request projectn/nemu!18

Riscv64 sv39

See merge request projectn/nemu!17

* CLINT should be included by MMIO virtual memory space in AM

Rvc bugfix

See merge request projectn/nemu!16

Intr difftest

See merge request projectn/nemu!15

* This may not hide the details of different ISA.

riscv64,exec: add fence.i

See merge request projectn/nemu!14

Rt thread

See merge request projectn/nemu!13

Freertos

See merge request projectn/nemu!12

* fix the cause number, the interrupt bit should set with
  0x8000000000000000, instead of 0x80000000

* `clint` is for Core-Level INTerrupt controller
* according to the privileged ISA manual, it is used to provide
  memory-mapped mtime and mtimcmp registers to generate M-mode timer
  interrupt

Rvc

See merge request projectn/nemu!11