Fix warmup phase statistics and multicore CPU handling

[konkanello]

• Fix ROI stats reporting: use roi_instr/roi_cycle for simulation phases and sim_instr/sim_cycle for warmup phases
• Fix end_phase() to only update roi_stats during simulation (not warmup)
• Only call end_phase for the CPU that actually finished
• Add trace file extensions to .gitignore (*.xz, *.gz, *.trace)
• Update default config to 2 cores for testing multicore scenarios

[maccoymerrell]

I would suggest retaining the roi_stat update for both warmup and sim phase in end_phase, that way roi_instr() and roi_cycle() can be used universally for printing out final stats at the end of any phase (should simplify your code in champsim.cc nicely).

The original intent behind this system was that roi would hold the final results of a phase while sim_stats would hold the in-progress phase stats. This made more sense when our original goal was to have many user-defined phases for the simulation. Warmup was the initial phase, and was treated differently.