x86/time: group time stamps into a structure

x86/time: group time stamps into a structure

If that had been done from the beginning, mistakes like the one
corrected in commit b64438c7c1 ("x86/time: use correct (local) time
stamp in constant-TSC calibration fast path") would likely never have
happened.

Also add a few "const" to make more obvious when things aren't expected
to change.

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