devel/*{-xtoolchain,}-gcc: Retire the old GCC external toolchain ports.

These have been supplanted by the devel/freebsd-gcc6 port which uses
separate flavors for each target architecture and bundles the
CROSS_TOOLCHAIN makefile.

Feedback from: jrm
Reviewed by: lwhsu, bapt
Differential Revision: https://reviews.freebsd.org/D30765