sh: mark end of BUG() implementation as unreachable

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@beast
Signed-off-by: Kees 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>