readelf/objdump: support for SFrame section

This patch adds support for SFrame in readelf and objdump. The arguments
of --sframe are optional for both readelf and objdump.

include/ChangeLog:

	* sframe-api.h (dump_sframe): New function declaration.

ChangeLog:

	* binutils/Makefile.am: Add dependency on libsframe for
	readelf and objdump.
	* binutils/Makefile.in: Regenerate.
	* binutils/doc/binutils.texi: Document --sframe=[section].
	* binutils/doc/sframe.options.texi: New file.
	* binutils/objdump.c: Add support for SFrame format.
	* binutils/readelf.c: Likewise.
	* include/sframe-api.h: Add new API for dumping .sframe
	section.
	* libsframe/Makefile.am: Add sframe-dump.c.
	* libsframe/Makefile.in: Regenerate.
	* libsframe/sframe-dump.c: New file.