lang/gcc6-aux: Add support for aarch64-*-freebsd*
The resultant compiler passes all test except:
- The stack-check related ones 
- c34005o and c34007i 
This should enable the full Ada framework, including Synth, on
FreeBSD/ARM64. I am not yet able to test this myself since I'm not part
of the wheel group on the aarch64 reference machines.
 I will look into providing the missing unwind support for aarch64
which would be something to push upstream to gcc
 I'll ask Adacore about these GNAT ICE (internal compiler errors) and
open GCC PRs if necessary since they could affect other aarch64 platforms.