The first patch was, under a different title and with a different
approach, already part of the prior series of the same subject.
The other two patches are new, resulting from me spotting further
room for improvement (or so I hope).

1: latch to-be-written TSC value early in rendezvous loop
2: yield to hyperthreads after updating TSC during rendezvous
3: avoid reading the platform timer in rendezvous functions

Jan