nvptx: Support '--with-multilib-list'

No change in behavior unless specifying it.

	gcc/
	* config.gcc: nvptx: Support '--with-multilib-list'.
	* config/nvptx/gen-multilib-matches.sh: Adjust.
	* configure.ac: Likewise.
	* configure: Regenerate.
	* doc/install.texi: Update.
	* doc/invoke.texi: Align.
	* config/nvptx/gen-multilib-matches-tests: Extend.