Fix up kernel stats, allow them to not be used as well.

Fix up kernel stats, allow them to not be used as well.

arch/alpha/ev5.cc:
    Fix up some stuff I missed in the last kernel stats checkin.
cpu/checker/cpu.cc:
    Allow the checker to disable its kernel stats.
cpu/cpu_exec_context.cc:
    Allow CPUExecContext to be created without kernelStats.
cpu/cpu_exec_context.hh:
    Allow CPUExecContext to be created without kernelStats.  Default usage leaves kernelStats on.

--HG--
extra : convert_revision : 8ed5bffd3a5b6275baa07fb4ea385eeab1a0456a