SystemInformation: get CPU speed on Windows when RDTSC fails

The CPU may have a TSC but reading it may fail, in which case we now always
fall back to reading the value from the registry. This is especially true
on Windows64 where we do not have the assembler instructions at all.

Change-Id: I5b1aafac6cd8df57ed8c1ac3bafcc8e092e3d4fb