support --with-multilib-list=@name for ARM

support --with-multilib-list=@name for ARM

Introduce @name as a means to specify alternate multilib profiles as
arguments to --with-multilib-list.

So far this is only implemented for ARM.

for  gcc/ChangeLog

	* config.gcc (tmake_file): Add name to tmake_file for
        each @name in --with-multilib-list on arm-*-* targets.
	* doc/install.texi (with-multilib-list): Document it.

From-SVN: r267320