Previously, OcCpuLib assumed all CPUs had the same core crystal clock frequency for their ART (24 Mhz). Xeon Scalable and Intel Atoms with Goldmont architecture have different frequencies (25 Mhz and 19.2 Mhz respectively). If the CPU supports reporting its frequency, use that. Otherwise, fallback to a value based on its model and family.

Cleaned up the logic for how we compute our CPU and FSB frequencies. For CPUs that support ART, use that instead of the TSC frequency.

Added documentation and renamed `OC_CPU_INFO::TSCFrequency` to `CPUFrequencyFromTSC` and added `CPUFrequencyFromART` for clarity. These are intermediate values used to compute `CPUFrequency`. `ARTFrequency` is now correctly set to the core crystal clock frequency rather than `CPUFrequencyFromART`, which it was previously.

Lastly, adjusted the `CPUFrequencyFromART` calculation to include the TSC offset in `MSR_IA32_TSC_ADJUST` if present. I'm not aware of which CPUs have a non-zero offset but it's part of Intel's calculation for computing the CPU frequency based on the ART.

This reverts commit f3c3dc7d.

These aren't available in any Macs to date but they are similar to the Xeon Ws used in the iMacPro1,1 and the upcoming MacPro7,1. (#6)

Closes acidanthera/bugtracker#307