1. add ALU(almost copy from Noop.ALU)
   remove jal/jalr/ret/call from ALU
   remove predictWrong from ALU(judged by brq now)
   remove bpuUpdateReq from ALU
2. add Redirect to ExuOutput
   it is connected to brq

1. remove ExuInput.redirect to ExuIO.redirect for input redirect
don't need waiting for function unit's in.valid.
2. remove ExuOutput.redirect for redirect is only generate by brq
and roq. ALU/BRU.bj instr only need generate target and send to
brq.

  ExuInput is used to flush the function unit itself.
  ExuOutput is used to flush other function units.
  Just ROB, bru(can exec jal/jalr/csr instrs) and alu(can exec
branch instrs) can generate redirect.

Use fake Icache to fetch 8 instructions per cycle.

FPU can run at 200+MHz on axu3cg board now :)

[IntToFloat]: Count leading zeros of positive and negative number in parallel because IntToFloat is in the critical path

FPUv0.1 can pass all rvf/rvd tests in both riscv-tests and berkeley-softfloat
Signed-off-by: NLinJiawei <linjiav@outlook.com>

* refer to the RAS prediction hints in RV32I in the RISC-V manual

This reverts commit 2dfc501e.

This reverts merge request !72

* if we disable dcache, the compilation will fail

* This can help to generate desired frequency of timer interrupt, no
  matter what frequency of the clock is.

* According to The RISC-V Instruction Set Manual Volume II:
Privileged Architecture, for instruction-fetch access or
page-fault exceptions on systems with variable-length instructions,
m/stval will contain the virtual address of the portion
of the instruction that caused the fault while m/sepc will
point to the beginning of the instruction.

* An instruciton fetch will set the A bit of the PTE, even this
  instruction will be flushed later due to branch mis-prediction.
  Although this will not cause correctness issue, it will introduce
  inconsistent behavior compared with NEMU.
* To run DiffTest while testing debian, a solution is to disable
  updating A bit and D bit in both NOOP and NEMU.

* this can reduce the idle time in Linux