i. e. the merge of r134227 from x86 to use smp_ipi_mtx spin lock not only
for smp_rendezvous_cpus() but also for the MD cache invalidation, TLB
demapping and remote register reading IPIs due to the following reasons:
- The cross-IPI SMP deadlock x86 otherwise is subject to can't happen on
  sparc64. That's because on sparc64, spin locks don't disable interrupts
  completely but only raise the processor interrupt level to PIL_TICK. This
  means that IPIs still get delivered and direct dispatch IPIs such as the
  cache invalidation etc. IPIs in question are still executed.
- In smp_rendezvous_cpus(), smp_ipi_mtx is held not only while sending an
  IPI_RENDEZVOUS, but until all CPUs have processed smp_rendezvous_action().
  Consequently, smp_ipi_mtx may be locked for an extended amount of time as
  queued IPIs (as opposed to the direct ones) such as IPI_RENDEZVOUS are
  scheduled via a soft interrupt. Moreover, given that this soft interrupt
  is only delivered at PIL_RENDEZVOUS, processing of smp_rendezvous_action()
  on a target may be interrupted by f. e. a tick interrupt at PIL_TICK, in
  turn leading to the target in question trying to send an IPI by itself
  while IPI_RENDEZVOUS isn't fully handled, yet, and, thus, resulting in a
  deadlock.

concurrent sending of IPIs by different CPUs is fatal. Therefore, hold the
reintroduced MD ipi_mtx also while delivering cross-traps via MI helpers,
i. e. ipi_{all_but_self,cpu,selected}().

instead of the BSP in cpu_mp_unleash(). This ensures that all APs actually
are started, when smp_started is no longer 0.

smp_cpus > 1 or nothing at all. This avoids races during boot causing IPIs
trying to be delivered to APs that in fact aren't up and running, yet.
While at it, move setting of the cpu_ipi_{selected,single}() pointers to
the appropriate delivery functions from mp_init() to cpu_mp_start() where
it's better suited and allows to get rid of the global isjbus variable.

the delays before completely disabling interrupts again in the CPU-specific
cross-trap delivery functions, previously giving other CPUs a window for
sending IPIs on their part. Actually, we now should be able to entirely get
rid of completely disabling interrupts in these functions. Such a change
needs more testing, though.

necessary for correctness, this avoids page faults when accessing the stack
of a foreign CPU as {s,}tick now is locked into the TLBs as part of static
kernel data. Hence, {s,}tick_get_timecount_mp() always execute as fast as
possible, avoiding jitter.