[SFrame-V3] gas: sframe: doc: testsuite: add new command line option --gsframe-3

This option will allow users to select emission of SFrame stack trace
information as per the SFrame version 3 specification.  Currenly, SFrame
version 3 is also the default.

In future, as SFrame evolves, similar command line args may be added for
future versions.

gas/
	* as.c (enum gen_sframe_version): New definition.
	(parse_args): Add option processing for --gsframe-3.
	* as.h (enum gen_sframe_version): New declaration.
	* doc/as.texi: Document the new option.
	* gen-sframe.c (sframe_set_version): Use enum gen_sframe_version
	as version.
	(output_sframe): Likewise.
gas/testsuite/
	* gas/cfi-sframe/cfi-sframe.exp: New test.
	* gas/cfi-sframe/cfi-sframe-common-1d.d: Test new command line
	option --gsframe-3.
	* gas/cfi-sframe/cfi-sframe-common-1d.s: Likewise.