panic: make function declarations visible

A few panic() related functions have a global definition but not
declaration, which causes a warning with W=1:

kernel/panic.c:710:6: error: no previous prototype for '__warn_printk' [-Werror=missing-prototypes]
kernel/panic.c:756:24: error: no previous prototype for '__stack_chk_fail' [-Werror=missing-prototypes]
kernel/exit.c:1917:32: error: no previous prototype for 'abort' [-Werror=missing-prototypes]

__warn_printk() is called both as a global function when CONFIG_BUG
is enabled, and as a local function in other configs. The other
two here are called indirectly from generated or assembler code.

Add prototypes for all of these.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnd Bergmann <[e-mail]>
Cc: Boqun Feng <[e-mail]>
Cc: Catalin Marinas <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Dennis Zhou <[e-mail]>
Cc: Eric Paris <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Helge Deller <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Michael Ellerman <[e-mail]>
Cc: Michal Simek <[e-mail]>
Cc: Palmer Dabbelt <[e-mail]>
Cc: Paul Moore <[e-mail]>
Cc: Pavel Machek <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Rafael J. Wysocki <[e-mail]>
Cc: Russell King <[e-mail]>
Cc: Tejun Heo <[e-mail]>
Cc: Thomas Bogendoerfer <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Waiman Long <[e-mail]>
Cc: Will Deacon <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

include/asm-generic/bug.h

···
  *
  * Use the versions with printk format strings to provide better diagnostics.
  */
-#ifndef __WARN_FLAGS
 extern __printf(4, 5)
 void warn_slowpath_fmt(const char *file, const int line, unsigned taint,
 		       const char *fmt, ...);
+extern __printf(1, 2) void __warn_printk(const char *fmt, ...);
+
+#ifndef __WARN_FLAGS
 #define __WARN()		__WARN_printf(TAINT_WARN, NULL)
 #define __WARN_printf(taint, arg...) do {				\
 		instrumentation_begin();				\
···
 		instrumentation_end();					\
 	} while (0)
 #else
-extern __printf(1, 2) void __warn_printk(const char *fmt, ...);
 #define __WARN()		__WARN_FLAGS(BUGFLAG_TAINT(TAINT_WARN))
 #define __WARN_printf(taint, arg...) do {				\
 		instrumentation_begin();				\

include/linux/panic.h

···
 
 extern bool crash_kexec_post_notifiers;
 
+extern void __stack_chk_fail(void);
+void abort(void);
+
 /*
  * panic_cpu is used for synchronizing panic() and crash_kexec() execution. It
  * holds a CPU number which is executing panic() currently. A value of