This port is basically identical to the existing <arch>-gcc ports
that use GCC6. However, they use flavors for the different
architectures instead of slave ports. It also includes the external
toolchain makefile and thus subsumes the <arch>-xtoolchain-gcc
port into the new packages.
This should permit us to have ports for different major versions of
GCC (e.g. a freebsd-gcc9 port).
aarch64-gcc6 disables LTO support to work with aaarch64-binutils
which builds a static ld.bfd binary by default which is unable to work