x86/debug: Define BUG() again for !CONFIG_BUG

The latest change to the BUG() macro inadvertently reverted the earlier
commit:

  b06dd879f5 ("x86: always define BUG() and HAVE_ARCH_BUG, even with !CONFIG_BUG")

... that sanitized the behavior with CONFIG_BUG=n.

I noticed this as some warnings have appeared again that were previously
fixed as a side effect of that patch:

  kernel/seccomp.c: In function '__seccomp_filter':
  kernel/seccomp.c:670:1: error: no return statement in function returning non-void [-Werror=return-type]
  ...

This combines the two patches and uses the ud2 macro to define BUG()
in case of CONFIG_BUG=n.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 9a93848fe7 ("x86/debug: Implement __WARN() using UD0")
Link: http://lkml.kernel.org/r/20170329211646.2707365-1-arnd@arndb.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/bug.h

@@ -23,7 +23,6 @@
 #define LEN_UD0		2
 
 #ifdef CONFIG_GENERIC_BUG
-#define HAVE_ARCH_BUG
 
 #ifdef CONFIG_X86_32
 # define __BUG_REL(val)	".long " __stringify(val)
@@ -64,6 +63,13 @@ do {									\
 
 #endif /* CONFIG_DEBUG_BUGVERBOSE */
 
+#else
+
+#define _BUG_FLAGS(ins, flags)  asm volatile(ins)
+
+#endif /* CONFIG_GENERIC_BUG */
+
+#define HAVE_ARCH_BUG
 #define BUG()							\
 do {								\
 	_BUG_FLAGS(ASM_UD2, 0);					\
@@ -72,8 +78,6 @@ do {								\
 
 #define __WARN_TAINT(taint)	_BUG_FLAGS(ASM_UD0, BUGFLAG_TAINT(taint))
 
-#endif /* CONFIG_GENERIC_BUG */
-
 #include <asm-generic/bug.h>
 
 #endif /* _ASM_X86_BUG_H */