[ARM] Ensure machine information structures aren't optimised away

Since the machine information structures are now static, the
compiler might optimise them away.  Mark them with
__attribute_used__ to prevent this occuring.
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

include/asm-arm/mach/arch.h

@@ -50,6 +50,7 @@ struct machine_desc {
  */
 #define MACHINE_START(_type,_name)			\
 static const struct machine_desc __mach_desc_##_type	\
+ __attribute_used__					\
  __attribute__((__section__(".arch.info.init"))) = {	\
 	.nr		= MACH_TYPE_##_type,		\
 	.name		= _name,