ARC: atomic: !LLSC: remove hack in atomic_set() for for UP

!LLSC atomics use spinlock (SMP) or irq-disable (UP) to implement
criticla regions. UP atomic_set() however was "cheating" by not doing
any of that so and still being functional.

Remove this anomaly (primarily as cleanup for future code improvements)
given that this config is not worth hassle of special case code.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>