Build proot for x86_64 as well

[dsnopek]

This will build proot for x86_64 as well

Unfortunately, for this to actually work, we also need:

1. A rootfs built for x86_64 (which is much trickier)
2. GABE to download a different rootfs depending on the current architecture (this is shouldn't be too hard to do)

[syntaxerror247]

Looks good. I tested by building locally and the x86_64 libraries were generated as expected. However, I think we should hold off on merging until Rootfs is updated.

[m4gr3d]

A rootfs built for x86_64 (which is much trickier)
GABE to download a different rootfs depending on the current architecture (this is shouldn't be too hard to do)

@dsnopek Any updates on these tasks?

[dsnopek]

No, sorry, I haven't had a chance to work on the rootfs build system changes