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

* IFU will send 64-bit fetch requests. Originally flash will only
  response with rdata = 0x00000000????????. For instrucitons aligned at
  4-byte address, IDU will select 0x00000000 as the target instruction.
* To fix this, we let flash reponse with 2 copies.
* Note that RVC instruction stream like 0xccbbbbbbbbaa in flash can not
  be handled correctly. The current solution only support non-RVC code
  in flash.

* NOTE: if we fetch and execute a store instruction from MMIO, currently
  DiffTest can not handle this situation.

Otherwise the upstream dmemXbar will not behave correctly.

This reverts commit 196971d7.

This reverts commit d336f385.

Otherwise dmemXbar will not behave correctly.