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>