Feature: add support for ARM64 architecture

[jayaddison]

Debian's 'Salsa' continuous integration build infrastructure cross-compiles packages by default for ARM64 -- and while packaging quadrilateralcowboy for Debian, one of those cross-compilation jobs failed due to an unknown CPU type. That's because the code here as it exists today isn't aware of ARM-based architectures on Linux).

Also: the codebase derives from the "original" Doom 3 release -- not Doom 3 BFG, which seems to have better cross-platform compatibility. However: people have worked on improving the code from the original release to support different architectures.

Among those people is Oliver McFadden, who added Doom 3 support for the ARM architecture in his dante fork of the game.

It seems possible to build upon that work to provide ARM64 support for quadrilateralcowboy - I'll offer a pull request related to this soon.