libobjc2: mark aarch64 as bad platform

Apparently some assembly has not been ported. This commit should fix
it:

https://github.com/gnustep/libobjc2/commit/b952a21abe8b0f9878b1ec5161ea530058512f4a#diff-e49aa4e60f8ccec1be46afc5e6ffcee5

But no release has been made by the gnustep team yet!