lang/gnatdroid-armv8: Mark broken on FreeBSD 8

This has never built on FreeBSD 8 after the base compiler was upgraded
from GNAT 4.6 to 4.7 and later 4.9. It fails will many c++ complaints
about multiple definition of __x86.get_pc_thunk.cx and consequential
errors. I'm tired of the cluster builders sending me an email about it.
I'm never going to fix it, so let's mark it broken.