Linux ARM64 natives support

[k8ieone]

This PR adds support for building ARM64 Linux natives. All of the libraries except SDL2 are cross-compiled. SDL2 is built in an ARM Docker container emulated using QEMU user-mode emulation.

I made sure the changes won't interfere with compiling on x86. When compiling on x86 the SDL2 build is skipped.

[Anuken]

Is it possible to move this somewhere else to avoid polluting the root directory?

[k8ieone]

Sure thing! I'd go for either the .github folder if GitHub doesn't mind or some other folder like .ci or something in that spirit.

Do you have a place in mind for it?

[cdeletre]

Nice work ! With your modifications I've been able to build libarc-freetypearm64.so, libarc-filedialogsarm64.so, libarcarm64.so and libsdl-arcarm64.so pack them into the jar and run it on my raspberry pi400. I'm not very used to this gradlew things and I don't know what I'm doing wrong but I had to patchelf the libsdl-arcarm64.so to add libGLEW.so as required (patchelf --add-needed libGLEW.so.2.2 backends/backend-sdl/libs/linuxarm64/libsdl-arcarm64.so). It seems that during the build -lGLEW is not used which could be the reason however I can't get it to build if I add it in backends/backend-sdl/build.gradle (line 130)

Also I'm looking at how to build a linux arm64 version that uses SDL2 + OpenGLES2 without GLEW instead of SDL2 + OpenGL + GLEW. I'm thinking of that because I contribute a lot in PortMaster, a project where we port games to linux arm device (console mostly) which don't have a display manager (we use SDL2 KMS/DRM with GLES). Any tip on this will be highly welcome :)

Again thanks @k8ieone for your work !