Update hwpmc on armv7 to handle overflow better

[andrew]

Summary

When testing hwpmc on arm64 we found the counter could overflow while
reading the event count. Handle this case in the armv7 code by also
checking if the overflow bit is set and incrementing the overflow
cound as needed.

Sponsored by: Innovate UK

[andrew]

Set the overflow when restoring register values. Start is just to start the counters counting.