arch: conditionally define smp_{mb,rmb,wmb}

That way arches can define the minimal versions and still #include
asm-generic for defaults (vs. defining defaults in arch code)

See new barrier.h in arc for usage !
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

include/asm-generic/barrier.h

@@ -55,17 +55,42 @@
 #endif
 
 #ifdef CONFIG_SMP
+
+#ifndef smp_mb
 #define smp_mb()	mb()
+#endif
+
+#ifndef smp_rmb
 #define smp_rmb()	rmb()
+#endif
+
+#ifndef smp_wmb
 #define smp_wmb()	wmb()
+#endif
+
+#ifndef smp_read_barrier_depends
 #define smp_read_barrier_depends()	read_barrier_depends()
+#endif
+
 #else
+#ifndef smp_mb
 #define smp_mb()	barrier()
+#endif
+
+#ifndef smp_rmb
 #define smp_rmb()	barrier()
+#endif
+
+#ifndef smp_wmb
 #define smp_wmb()	barrier()
+#endif
+
+#ifndef smp_read_barrier_depends
 #define smp_read_barrier_depends()	do { } while (0)
 #endif
 
+#endif
+
 #ifndef set_mb
 #define set_mb(var, value)  do { (var) = (value); mb(); } while (0)
 #endif