Update the armv6 tlb handling functions to detect if it is running on
hardware that supports the mp extensions. If so it should use the broadcast
tlb invalidate instructions as other CPUs or devices may need to know about
the invalidation.

To simplify the code have the compiler optimise out the else case when not
builing for Cortex-A8.

Sponsored by: ABT Systems Ltd
Differential Revision: https://reviews.freebsd.org/D8092