x86: do not enable TSC notifier if we don't need it

Impact: crash on non-TSC-equipped CPUs

Don't enable the TSC notifier if we *either*:

1. don't have a CPU, or
2. have a CPU with constant TSC.

In either of those cases, the notifier is either damaging (1) or useless(2).

From: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>