x86/time: latch to-be-written TSC value early in rendezvous loop

x86/time: latch to-be-written TSC value early in rendezvous loop

To reduce latency on time_calibration_tsc_rendezvous()'s last loop
iteration, read the value to be written on the last iteration at the end
of the loop body (i.e. in particular at the end of the second to last
iteration).

On my single-socket 18-core Skylake system this reduces the average loop
exit time on CPU0 (from the TSC write on the last iteration to until
after the main loop) from around 32k cycles to around 29k (albeit the
values measured on separate runs vary quite significantly).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>