MFC: revision 208638

MFC: revision 208638

  - Add an integer argument to idle to indicate how likely we are to wake
    from idle over the next tick.
  - Add a new MD routine, cpu_wake_idle() to wakeup idle threads who are
    suspended in cpu specific states.  This function can fail and cause the
    scheduler to fall back to another mechanism (ipi).
  - Implement support for mwait in cpu_idle() on i386/amd64 machines that
    support it.  mwait is a higher performance way to synchronize cpus
    as compared to hlt & ipis.
  - Allow selecting the idle routine by name via sysctl machdep.idle.  This
    replaces machdep.cpu_idle_hlt.  Only idle routines supported by the
    current machine are permitted.

git-svn-id: svn://svn.freebsd.org/base/stable/8@214591 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f