General fixes for Sampling CPU in full system mode, and serialization of sampling CPU

General fixes for Sampling CPU in full system mode, and serialization of sampling CPU

cpu/intr_control.cc:
    Fix the reference to the cpu, to look up which cpu is being used
    In sampling mode can't use an absolute pointer to the cpu, use the
    exeContexts vector
cpu/intr_control.hh:
    Add two new functions to simplify MP interrupts, fix it for sampling CPU model

--HG--
extra : convert_revision : a69cdbb81e6aefa3fd5385416713c689300bbea8