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>

arch/arc/include/asm/atomic-spinlock.h

@@ -3,12 +3,10 @@
 #ifndef _ASM_ARC_ATOMIC_SPLOCK_H
 #define _ASM_ARC_ATOMIC_SPLOCK_H
 
-#ifndef CONFIG_SMP
-
- /* violating atomic_xxx API locking protocol in UP for optimization sake */
-#define arch_atomic_set(v, i) WRITE_ONCE(((v)->counter), (i))
-
-#else
+/*
+ * Non hardware assisted Atomic-R-M-W
+ * Locking would change to irq-disabling only (UP) and spinlocks (SMP)
+ */
 
 static inline void arch_atomic_set(atomic_t *v, int i)
 {
@@ -30,13 +28,6 @@ static inline void arch_atomic_set(atomic_t *v, int i)
 
 #define arch_atomic_set_release(v, i)	arch_atomic_set((v), (i))
 
-#endif
-
-/*
- * Non hardware assisted Atomic-R-M-W
- * Locking would change to irq-disabling only (UP) and spinlocks (SMP)
- */
-
 #define ATOMIC_OP(op, c_op, asm_op)					\
 static inline void arch_atomic_##op(int i, atomic_t *v)			\
 {									\