Remove arm/arm as a valid target.

TARGET=arm now defaults to TARGET_ARCH=armv7
TARGET_ARCH=arm is no longer valid.

Bump __FreeBSD_version to 1300073

Tested with make universe. Any stale LINT-V5 config files remaining in the tree
will fail the universe build. However, LINT-V5 was removed in r355119.

This retirement has been planned since last summer. The armv5 port is fragile:
it works OK for some peeople, and fails badly for others. There's a number of
subtle bugs in busdma, pmap and other MD parts of thee system that present
themselves under load or in unusual circumstances (like fsck after a
crash). stable/8, branched 10 years ago, was the last reliable release. Since
the support burden is larger then the benefit, the consensus view is armv5
should be removed from the tree.

Discussed with: arm@ mailing list and arm developer community.