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

files/patch-aarch64-support,
files/patch-libgcc-config-arm-unwind-arm, and
files/patch-x86-64-fix-m16

which andreast@ and me pushed upstream.

On the other hand we gain bin/gcov-dump6 and man pages for gcov-dump6
and gcov-tool6.

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.