Preliminary armv7 support

[Warner Losh]

Greetings,

In support of FCP-0100, I've created two reviews for implementing the new
MACHINE_ARCH armv7 (and by extension TARGET_ARCH). The first one implements
the user space portion of the problem, while the second changes the kernel
builds.

https://reviews.freebsd.org/D12010
https://reviews.freebsd.org/D12027

Please give any feedback in those reviews. You'll need a newest -current
since a number of cleanups to the build were done as part of this effort
and committed separately.

Warner