arm: Move contents of atomic-v6.h into atomic.h

Previously we had an armv4/v5 and armv6/v7 implementation of the atomic
operations. As we have removed armv4/v5 support we can now merge the
armv6/v7 code into atomic.h

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D41135