Tweak the low-level MI SMP code some:

• Use cpu_spinwait() in the spin loops in stop_cpus(), restart_cpus(), and smp_rendezvous_action().
• Remove unneeded acq memory barriers in stop_cpus(), restart_cpus(), and smp_rendezvous_action().
• Add an additional synch point in smp_rendezvous() to ensure that all the CPUs will always see an up-to-date value of smp_rv_setup_func.

Reviewed by: attilio
Approved by: re (kensmith)
Tested on: alpha, amd64, i386, sparc64 SMP (for several years)