ARC: atomic64: LLSC: elide unused atomic_{and,or,xor,andnot}_return

This is a non-functional change since those wrappers are not
used in kernel sources at all.

Link: http://lists.infradead.org/pipermail/linux-snps-arc/2018-August/004246.html
Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>