powerpc: Convert BUG() to use unreachable()

Use the new unreachable() macro instead of for(;;);

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <paulus@samba.org>
CC: linuxppc-dev@ozlabs.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/bug.h

@@ -68,7 +68,7 @@
 		_EMIT_BUG_ENTRY					\
 		: : "i" (__FILE__), "i" (__LINE__),		\
 		    "i" (0), "i"  (sizeof(struct bug_entry)));	\
-	for(;;) ;						\
+	unreachable();						\
 } while (0)
 
 #define BUG_ON(x) do {						\