Add in checker. Supports dynamically verifying the execution of instructions, as...

Add in checker.  Supports dynamically verifying the execution of instructions, as well as limited amount of control path verification.  It will verify anything within the program, but anything external (traps, interrupts, XC) it assumes is redirected properly by the CPU.  Similarly it assumes the results of store conditionals, uncached loads, and instructions marked as "unverifiable" are correct from the CPU.

base/traceflags.py:
build/SConstruct:
cpu/SConscript:
cpu/cpu_models.py:
    Add in Checker.
cpu/base.cc:
    Add in checker support.  Also XC status starts off as suspended.
cpu/base.hh:
    Add in checker.

--HG--
extra : convert_revision : 091b5cc83e837858adb681ef0137a0beb30bd1b2