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

* previously it may trigger an assertion fail if the number of basic
  blocks of a program is less than TOP_N
* now if that is the case, we will display statistic of an empty basic
  block

Reg spilling

See merge request projectn/nemu!54

* this is wrong with a translation engine
* now we can pass microbench and fceux

* previously we use rtl_andi(s0, s1, 0xfffffffful), but 0xffffffff can
  not be loaded correctly by load_imm() in the rv64 engine

* else it may be overwritten with the runtime bbl

* they are only valid during the translation of a single instruction

* now we can first check whether the second rtlreg is already mapped

* this is undefined behavior, which should be avoided