ARC: unwind: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings (Building: haps_hs_defconfig arc):

arch/arc/kernel/unwind.c:827:20: warning: this statement may fall through [-Wimplicit-fallthrough=]
arch/arc/kernel/unwind.c:836:20: warning: this statement may fall through [-Wimplicit-fallthrough=]

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>