sh: mark end of BUG() implementation as unreachable

When building the sh architecture, the compiler doesn't realize that
BUG() doesn't return, so it will complain about functions using BUG()
that are marked with the noreturn attribute:

   lib/string.c: In function 'fortify_panic':
>> lib/string.c:986:1: warning: 'noreturn' function does return
    }
    ^

Link: http://lkml.kernel.org/r/20170627192050.GA66784@beastSigned-off-by: NKees Cook <keescook@chromium.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: Daniel Micay <danielmicay@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/sh/include/asm/bug.h

@@ -48,6 +48,7 @@ do {							\
 		   "i" (__FILE__),			\
 		   "i" (__LINE__), "i" (0),		\
 		   "i" (sizeof(struct bug_entry)));	\
+	unreachable();					\
 } while (0)
 
 #define __WARN_FLAGS(flags)				\