ARC: cpu_relax() to be compiler barrier even for UP

cpu_relax() on ARC has been barrier only for SMP (and no-op for UP). Per
recent discussions, it is safer to make it a compiler barrier
unconditionally.

Link: http://lkml.kernel.org/r/53A7D3AA.9020100@synopsys.com


Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>