JNBullet does not have Mac aarch64 target

[jrddunbr]

As part of the debugging I've been doing to get Terasology to build on M1, I've discovered that this project does not have a macOS aarch64 target, only i386 and amd64. This is a complete blocker to running Terasology on M1 as far as I can tell, unless you run all of the Terasology build steps in macOS amd64 compatibility mode (which I'd rather not, and have not tested).

See MovingBlocks/Terasology#5055 for context.

(I guess I'll have to stop being lazy and plug in my Linux desktop to play Terasology, oh well!)