ARM: remove __bad_xchg definition

We want link errors if xchg() is called for a variable size we do not
support.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/include/asm/cmpxchg.h

@@ -94,6 +94,7 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
 		break;
 #endif
 	default:
+		/* Cause a link-time error, the xchg() size is not supported */
 		__bad_xchg(ptr, size), ret = 0;
 		break;
 	}