* 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

* the LSB of funct7 may be "1" due to the shift amount can be >= 32

* the src should be first sign-extended

* Now we can run PAL over Nanos-lite, but it gets stuck while battling.
  It seems mul/div instructions are wrong.