Fix for FS O3CPU compile ... missing forward class declaration/header file after...

Fix for FS O3CPU compile ... missing forward class declaration/header file after files got split for ISA-independence

src/cpu/o3/alpha/thread_context.hh:
    Use 'this' when accessing cpu
src/cpu/o3/cpu.hh:
    add numActiveThreds function
src/cpu/o3/thread_context.hh:
    forward class declarations
src/cpu/o3/thread_context_impl.hh:
    add quiesce event header file
src/cpu/thread_context.hh:
    add exit() function to thread context (read comments in file)
src/sim/syscall_emul.cc:
    adjust exitFunc syscall

--HG--
extra : convert_revision : 323dc871e2b4f4ee5036be388ceb6634cd85a83e