powerpc: add a best-effort SMP time base sync for G5's that need it

There's no timebase freeze platform routine registered on my dual 2.3GHz
G5 PPC970FX Apple PowerMac.

For platforms without an explicit timebase freeze/unfreeze, we'll have to
make do with what we have - which for now is an explicit hand-crafted
spinlock/rendezvous method.

• For existing platforms, they'll still continue to clock freeze / rendezvous; albeit with some stronger atomic bits now (from jhibbits@.)
• Instead of the fallback being "no timesync", implement a best-effort one which does a similar rendezvous barrier between BSP and APs, but instead of freeze/unfreeze the first instruction after the CPUs all register they're ready is to set the timebase.

This has resulted in many reboots of my Powermac G5 dual-socket device
correctly starting and running in SMP mode.

Differential Revision: https://reviews.freebsd.org/D54821
Reviewed by: jhibbits