sched/clock/x86: Mark sched_clock() noinstr

In order to use sched_clock() from noinstr code, mark it and all it's
implenentations noinstr.

The whole pvclock thing (used by KVM/Xen) is a bit of a pain,
since it calls out to watchdogs, create a
pvclock_clocksource_read_nowd() variant doesn't do that and can be
noinstr.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230126151323.702003578@infradead.org