objdump/readelf: sframe: emit marker for FREs with mangled RA

In the textual dump of the SFrame section, when an SFrame FRE recovers a
mangled RA, use string "[s]" in the output to indicate that the return
address is a signed (mangled) one.

ChangeLog:

        * libsframe/sframe-dump.c (dump_sframe_func_with_fres): Postfix
	with "[s]" if RA is signed with authorization code.