r440699 broke xorg on aarch64 (rpi3 and alike)

[Gergely Czuczy]

Hello,

r440699 [1] made mesa-dri unconditionally depend on s2tc, which 
unconditionally depends on nvidia-texture-tools. nvidia-texture-tools is 
not supported on aarch64, which is the platform of rpi3.

Could you please make sure xorg works on aarch64 as well? We don't have 
a lot of alternatives on that platform. Making the non-aarch64 
compatible ports optional would be a good start.

Also, let me know if you need a PR for this.

Best regards,
Gergely

[1] https://svnweb.freebsd.org/ports?view=revision&revision=440699