sched: allow architectures to specify sched_clock_stable

Allow CONFIG_HAVE_UNSTABLE_SCHED_CLOCK architectures to still specify
that their sched_clock() implementation is reliable.

This will be used by x86 to switch on a faster sched_clock_cpu()
implementation on certain CPU types.

Signed-off-by: Ingo Molnar <mingo@elte.hu>