Add freebsd-gcc6 port.

This port is basically identical to the existing <arch>-gcc ports
that use GCC6. However, it uses 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
with LTO.

Reviewed by: imp, brooks
Approved by: portmgr (swills)
Differential Revision: https://reviews.freebsd.org/D22147