Introduce an MI cpu synchronization API, redo the SMP AP startup code,

Introduce an MI cpu synchronization API, redo the SMP AP startup code,
and start cleaning up deprecated IPI and clock code.  Add a MMU/TLB page
table invalidation API (pmap_inval.c) which properly synchronizes page
table changes with other cpus in SMP environments.

    * removed (unused) gd_cpu_lockid
    * remove confusing invltlb() and friends, normalize use of cpu_invltlb()
      and smp_invltlb().
    * redo the SMP AP startup code to make the system work better in
      situations where all APs do not startup.
    * add memory barrier API, cpu_mb1() and cpu_mb2().
    * remove (obsolete, no longer used) old IPI hard and stat clock forwarding
      code.
    * add a cpu synchronization API which is capable of handling multiple
      simultanious requests without deadlocking or livelocking.
    * major changes to the PMAP code to use the new invalidation API.
    * remove (unused) all_procs_ipi() and self_ipi().
    * only use all_but_self_ipi() if it is known that all AP's started up,
      otherwise use a mask.
    * remove (obsolete, no longer usde) BETTER_CLOCK code
    * remove (obsolete, no longer used) Xcpucheckstate IPI code

Testing-by: David Rhodus and others