Tracing now works for upto 4 threads. Easy change to get it to work for more, but...

Tracing now works for upto 4 threads. Easy change to get it to work for more, but I don't have any test handy to test it.

cpu/trace/reader/m5_reader.cc:
    Add thread num.
cpu/trace/trace_cpu.cc:
    Increase thread count to 4, might want to make this a parameter (but it only really costs us storage).

--HG--
extra : convert_revision : 97cd7843668a3ef85aad06e3180dc04d2ca30ac1