clock/tsc: Detect invariant TSC

According to Intel's description:
"The invariant TSC will run at a constant rate in all ACPI P-, C-. "
 and T-states. ..."

The difference between invariant TSC and constant TSC is that
invariant TSC is not affected by frequency changes and deep ACPI
C-state.

Constant TSC could be detected based on the CPU model (Intel has
the model list, while there is no information from AMD's document);
constant TSC is not detected yet.
net: Use tsc_invariant when it is necessary; mainly in time measure