rs6000: Add support for powerpc64le-unknown-freebsd

This implements support for powerpc64le architecture on FreeBSD.  Since
we don't have powerpcle (32-bit), I did not add support for powerpcle
here. This remains to be changed if there is powerpcle support in the
future.

2020-12-15  Piotr Kubaj  <pkubaj@FreeBSD.org>

gcc/
	* config.gcc (powerpc*le-*-freebsd*): Add.
	* configure.ac (powerpc*le-*-freebsd*): Ditto.
	* configure: Regenerate.
	* config/rs6000/freebsd64.h (ASM_SPEC_COMMON): Use ENDIAN_SELECT.
	(DEFAULT_ASM_ENDIAN): Add little endian support.
	(LINK_OS_FREEBSD_SPEC64): Ditto.