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>