Snapshot difftest

Makefile: support snapshot on the remote machine

* add EMU_FLAGS including -s, -b, -e, --load-snapshot
* support snapshot on the remote machine with the same usage as if on local machine

Snapshot

* we may call stream.save() without stream.init() if the simulation ends
  within 10s

* But difftest is disabled when running with a snapshot. This will be
  fixed in the future.

* by default, snapshot will be saved every 10s

Tune compile options

* this can reduce the build time of the slow object files, which
  introduces little performance loss

* The -g option will generate debug information for every .cpp file.
  This information will be merged togather during linking, which is time
  consuming.

statistics.py: can count the number of lines of verilog code and log of each module

Unit-test: update code

Fix lsu store logic

Dev bpu rebase tage

BPU: fix s3 instrValid and s1 valid

Vga