Tag all armv7 kernels as such in their machine config line.

Transition all boards that support arm cortex CPUs to armv7. This
leaves two armv6 kernels in the tree. RPI-B, which uses the BCM2835
which has a ARM1176 core, and VERSATILEPB, which is a qemu board setup
around the time RPI-B went in. Copy std.armv6 to std.armv7, even
though that duplicates a lot of stuff. More work needs to be done to
sort out the duplication.

Differential Revision: https://reviews.freebsd.org/D12027