lang/gcc6-aux: Add support for aarch64-*-freebsd*

The resultant compiler passes all test except:

• The stack-check related ones [1]
• c34005o and c34007i [2]

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.

[1] I will look into providing the missing unwind support for aarch64

which would be something to push upstream to gcc

[2] I'll ask Adacore about these GNAT ICE (internal compiler errors) and

open GCC PRs if necessary since they could affect other aarch64
platforms.