[kernel][sched][x86] Use monitor/mwait when idle

When rescheduling idle cpus, use monitor/mwait instead of relying on
IPIs. This change adds some reschedule-specific arch hooks, instead of
relying on the arch IPI hooks.

The x86 percpu state includes a variable that tracks whether the cpu is
running the idle thread. The idle thread monitors and mwaits on that
variable. Then other threads can reschedule the idle cpu by clearing
the monitored variable.

ZX-1713
ZX-1293 #done

Change-Id: I5f7bf073e3e5b6e1e5fa4febc412f52f40773e2d

[x86][asm] Convert comments from # to //

Change-Id: I3aa8035a9748262373409444d74c6d9cf465c7ec

[kernel][x86] Remove arch/x86/64/ subdirectory

Zircon only supports x86-64.
The historical subdirectory split just makes it harder to find all the code.

Change-Id: I7c828660bd1d4d277718aa74f35a3fab5f2333ea