arm64: Add support for /proc/sys/debug/exception-trace

This patch allows setting of the show_unhandled_signals variable via
/proc/sys/debug/exception-trace. The default value is currently 1
showing unhandled user faults (undefined instructions, data aborts) and
invalid signal stack frames.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Nicolas Pitre <nico@linaro.org>
Acked-by: Olof Johansson <olof@lixom.net>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>

kernel/sysctl.c

@@ -1544,7 +1544,7 @@ static struct ctl_table fs_table[] = {
 
 static struct ctl_table debug_table[] = {
 #if defined(CONFIG_X86) || defined(CONFIG_PPC) || defined(CONFIG_SPARC) || \
-    defined(CONFIG_S390) || defined(CONFIG_TILE)
+    defined(CONFIG_S390) || defined(CONFIG_TILE) || defined(CONFIG_ARM64)
 	{
 		.procname	= "exception-trace",
 		.data		= &show_unhandled_signals,