config.gcc (powerpc*-*-linux*): Add support for little-endian multilibs to big-endian target and vice versa.

	* config.gcc (powerpc*-*-linux*): Add support for little-endian
	multilibs to big-endian target and vice versa.
	* config/rs6000/t-linux64: Use := assignment on all vars.
	(MULTILIB_EXTRA_OPTS): Remove fPIC.
	(MULTILIB_OSDIRNAMES): Specify using mapping from multilib_options.
	* config/rs6000/t-linux64le: New file.
	* config/rs6000/t-linux64bele: New file.
	* config/rs6000/t-linux64lebe: New file.

From-SVN: r202190