arc: export symbol for save_stack_trace() in stacktrace.c

Need export its symbol just like other architectures done, or can not
pass compiling with allmodconfig, the related error:

    MODPOST 2994 modules
  ERROR: "save_stack_trace" [kernel/backtracetest.ko] undefined!
  ERROR: "save_stack_trace" [drivers/md/persistent-data/dm-persistent-data.ko] undefined!
Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/kernel/stacktrace.c

@@ -246,4 +246,5 @@ void save_stack_trace(struct stack_trace *trace)
 	/* Pass NULL for task so it unwinds the current call frame */
 	arc_unwind_core(NULL, NULL, __collect_all, trace);
 }
+EXPORT_SYMBOL_GPL(save_stack_trace);
 #endif