x86/time: correctly honor late clearing of TSC related feature flags

x86/time: correctly honor late clearing of TSC related feature flags

As such clearing of flags may have an impact on the selected rendezvous
function, defer the establishing of a rendezvous function other than
the initial default one (std) until after all APs have been brought up.

But don't allow such feature flags to be cleared during CPU hotplug:
Platform and local system times may have diverged significantly by
then, potentially causing noticeably (even if only temporary) strange
behavior. As we're anyway expecting only sufficiently similar CPUs to
appear during hotplug, this shouldn't be introducing new limitations.

Reported-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>