coretemp(4): Switch to smp_rendezvous_cpus().

Use of smp_rendezvous_cpus() instead of sched_bind() allows to not
block indefinitely if target CPU is running some thread with higher
priority, while all we need is single rdmsr/wrmsr instruction call.
I guess it should also be much cheaper than full thread migration.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.

(cherry picked from commit 74f80bc1af2ffd56ec290f610c80e46f768731a0)