add in an init() callback for CPU's so that no stats are accessed prior to the end...

add in an init() callback for CPU's so that no stats are accessed prior to the end of the build process.  (Done by doing the registerExecContext() calling sequence in the init() process rather than the create() process).

cpu/simple_cpu/simple_cpu.cc:
cpu/simple_cpu/simple_cpu.hh:
    same thing for simple cpu's.

--HG--
extra : convert_revision : aac9f91742866fb26f8cace622f9b88454a69662