gas: add new command line option --gsframe

When --gsframe is specified, the assembler will generate a .sframe
section from the CFI directives in the assembly.

ChangeLog:

	* gas/as.c (parse_args): Parse args and set flag_gen_sframe.
	* gas/as.h: Introduce skeleton for --gsframe.
	* gas/doc/as.texi: document --gsframe.