config.gcc (visium-*-*): Enable --with-cpu option, accept gr5 and gr6 as possible values, defaulting to gr5.

	* config.gcc (visium-*-*): Enable --with-cpu option, accept gr5 and
	gr6 as possible values, defaulting to gr5.  Set target_cpu_default2.
	* config/visium/visium.h (OPTION_DEFAULT_SPECS): Define.
	(TARGET_CPU_gr5): Likewise.
	(TARGET_CPU_gr6): Likewise.
	(MULTILIB_DEFAULTS): Likewise.
	* config/visium/t-visium (MULTILIB_OPTIONS): Request distinct variants
	for mcpu=gr5 and mcpu=gr6.
	(MULTILIB_DIRNAMES): Adjust accordingly.

From-SVN: r227728