Add new sparc options to control instruction availability.

Add new sparc options to control instruction availability.

gas/

	* config/tc-sparc.c (hwcap_allowed): New.
	(struct sparc_arch): New field 'hwcap_allowed' containing a bitmask
	of F_FOO flags which are enabled by the particular arch setting.
	Add new options that provide explicit access to new instructions.
	(md_parse_option): Only bump max_architecture if the requested one
	is larger, or this is the first explicit request.
	(get_hwcap_name): New function.
	(sparc_ip): Validate that hwcaps used by an instruction have actually
	been enabled.
	* doc/c-sparc.texi: Document new sparc options.