libpmc: Prefer fixed counters in Intel event aliases.

It is recommended by Intel to use fixed counters when possible to
leave programmable ones for different events.

This fixes pmc stat, which tries to count 6 events same time, while
many Intel CPUs have only 4 programmable counters.

MFC after: 1 month

(cherry picked from commit e144cd92bb150b5636f97af3c1c89da49edf8ac8)