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.