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