Update to the GCC 6.4.0 upstream release which carries a number of fixes.
This now comes as an xz archive, so update USES=tar accordingly.
Also this now includes
which andreast@ and me pushed upstream.
On the other hand we gain bin/gcov-dump6 and man pages for gcov-dump6
Finally, as with lang/gcc5 and lang/gcc6-devel, remove headers being
created by GCC's fixincludes machinery from the installation / packaging
to avoid breakage when FreeBSD's headers are changing afterwards.