Fixed issue #4: Advanced Power Management Features

- Move detection of constant_tsc to common; it is spec'd in Intel docs
- Fix broken interpretation of EAX in leaf 80000000h: it shows max
  800000xx-value, not max xx-value. This causes, for example, to seek
  for extended features on Pentium II, where the extended leafs aren't
  supported by the CPU at all.
  This is only in common feature detection. AMD detection is fine.
- Add detection of a few new features in AMD leaf 80000007h: cbp (core
  performance boost), aperfmperf (APERF/MPERF MSRs supported),
  pfi (processor feedback interface) and pa (processor accumulator).

"make test" is broken right now, to be fixed in a subsequent commit.