alpha: Convert BUG() to use unreachable()

Use the new unreachable() macro instead of for(;;);
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
CC: Richard Henderson <rth@twiddle.net>
CC: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
CC: linux-alpha@vger.kernel.org
Signed-off-by: Matt Turner <mattst88@gmail.com>

arch/alpha/include/asm/bug.h

@@ -13,7 +13,8 @@
 		"call_pal %0  # bugchk\n\t"				\
 		".long %1\n\t.8byte %2"					\
 		: : "i"(PAL_bugchk), "i"(__LINE__), "i"(__FILE__));	\
-	for ( ; ; ); } while (0)
+	unreachable();							\
+  } while (0)
 
 #define HAVE_ARCH_BUG
 #endif