ARC: xchg: !LLSC: remove UP micro-optimization/hack

It gets in the way of cleaning things up and is a maintenance
pain-in-neck !
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@kernel.org>

arch/arc/include/asm/cmpxchg.h

@@ -113,15 +113,9 @@ static inline unsigned long __xchg(unsigned long val, volatile void *ptr,
  *  - For !LLSC, cmpxchg() needs to use that lock (see above) and there is lot
  *    of  kernel code which calls xchg()/cmpxchg() on same data (see llist.h)
  *    Hence xchg() needs to follow same locking rules.
- *
- * Technically the lock is also needed for UP (boils down to irq save/restore)
- * but we can cheat a bit since cmpxchg() atomic_ops_lock() would cause irqs to
- * be disabled thus can't possibly be interrupted/preempted/clobbered by xchg()
- * Other way around, xchg is one instruction anyways, so can't be interrupted
- * as such
  */
 
-#if !defined(CONFIG_ARC_HAS_LLSC) && defined(CONFIG_SMP)
+#ifndef CONFIG_ARC_HAS_LLSC
 
 #define arch_xchg(ptr, with)		\
 ({					\