ARM, locking/atomics: Implement _relaxed variants of atomic[64]_{inc,dec}

ARM, locking/atomics: Implement _relaxed variants of atomic[64]_{inc,dec}

Now that the core code supports acquire/release/relaxed versions of
the atomic_inc family, implement only the _relaxed flavours in the ARM
backend so that we get all of the others for free.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1444227038-12533-1-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>