x86/time: introduce and use rdtsc_ordered()

x86/time: introduce and use rdtsc_ordered()

Matching Linux commit 03b9730b76 ("x86/asm/tsc: Add rdtsc_ordered() and
use it in trivial call sites") and earlier ones it builds upon, let's
make sure timing loops don't have their rdtsc()-s re-ordered, as that
would harm precision of the result (values were observed to be several
hundred clocks off without this adjustment).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Joao Martins <joao.m.martins@oracle.com>