lang/gcc6-aux: Add unwind support to aarch64-freebsd

Together with a gcc devel patch to expmed.c to fix the two GNAT
internal compiler errors seen on the ACATS testsuite, the new signal
frame unwinder the remaining testsuite errors. The Ada compiler on
aarch64-*-freebsd* is now perfect.

Unfortunately the revision bump causes an unnecessary rebuild on x86, but
that can't be avoid.

The unwind support will be pushed upstream to GCC.