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
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