This is a patch fixing the ThreadContext suspension / activation described in http://www.mail-archive.com/m5-dev@m5sim.org/msg07307.html and implementing the nanosleep() syscall (albeit without the signal-relevant parts) for AlphaLinux.

Changes in O3:
- The tick scheduling was removed from activateContext() and moved into activateThread(). It seems more natural since activateContext() either calls activateThread() or schedules it. In the case of scheduling there is no need to schedule ticks prematurely.
- suspendContext() and haltContext() check the number of active threads before setting CPU _status to Idle.