Improve cpu_idle():

Improve cpu_idle():
o   cpu_idle_hook is expected to be called with interrupts
    disabled and re-enables interrupts on return.
o   sync with x86: don't idle when the CPU has runnable tasks
o   have callers of ia64_call_pal_static() disable interrupts
    and re-enable interrupts.
o   add, but compile-out, support for idle mode. This will be
    enabled at some later time, after proper testing.