build: partially fix configure on ARM

V8 on ARM requires that armv7 is set. We don't have a good way to detect the
CPU model right now so we pick a default and hope that it works okay for the
majority of people.

Non-scientific sampling - the ARM hardware I have lying around the house -
suggests that ARMv5 and ARMv6 are still most common so armv7=0 it is.

This obviously needs to be revisited sometime in the future.