[NTOS:KE] Don't loop indefinitely trying to figure out the CPU frequency. CORE-14419

[NTOS:KE] Don't loop indefinitely trying to figure out the CPU frequency. CORE-14419

Previously, we would keep sampling the CPU frequency until two subsequent
samples differed by at most 1 MHz. This could take several seconds, and would
unnecessarily delay boot.
Instead, if sampling is too unreliable, just give up and calculate the average
frequency from 10 samples. This is no worse than picking the frequency that
just happened to be returned twice in a row.

The fact that this method of sampling fails could indicate that there's a
problem with our performance counter implementation or timer interrupt,
but that's a separate issue...