Now that we can detect the Cortex-A8 properly, fix the event list
according to the Cortex-A8 TRM r3p2 section 3.2.49.
The A8 list differs from the "ARM-v7 common" list, given the A8
was an earlier model.

There is still more work to be done for other Cortex-Ax version as
andrew points out, but I am just trying to fix A8 for now for teaching.

MFC after: 2 weeks
Sponsored by: DARPA/AFRL
Obtained from: Cambridge/L41
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D3876