Complain about missing CONFIG_ARMV*

Without this a user may encounter the following error.

arch/arm/include/asm/atomic.h:38:2: error: implicit declaration of function ‘smp_mb’

With the #error pragma a user is a least pointed to the specific
section in the file that's causing the problem

arch/arm/include/asm/atomic.h:21:2: error: #error ARM architecture version (CONFIG_ARMV*) not set or unsupported.
Acked-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

arch/arm/include/asm/atomic.h

@@ -16,6 +16,10 @@ typedef struct {
 
 #define smp_mb() __asm__ __volatile__ ("mcr p15, 0, %0, c7, c10, 5"	: : "r" (0) : "memory")
 
+#else
+
+#error ARM architecture version (CONFIG_ARMV*) not set or unsupported.
+
 #endif
 
 static inline int atomic_read(const atomic_t *v)