I have disabled building SDL with headless EGL, because
SDL_config_minimal.h doesn't work with EGL, and I don't know how to
implement a an SDL config that would work with EGL.
I have verified that this works in a separate project, and that it
compiles in this project.
```
$ ./build.sh -e release
$ find ./out/ -name "*EGL*"
./out/cmake-release/filament/backend/CMakeFiles/backend.dir/src/opengl/platforms/PlatformEGL.cpp.o
./out/cmake-release/filament/backend/CMakeFiles/backend.dir/src/opengl/platforms/PlatformEGLHeadless.cpp.o
./out/cmake-release/libs/bluegl/CMakeFiles/bluegl.dir/src/BlueGLLinuxEGL.cpp.o
```
Some clients need to link against both Filament and OpenGL. This can lead to symbol clashes under certain circumstances. As a fix, prefix all of our `bluegl` symbols with `bluegl_`. Previously we made this naming change for Windows, but now we'll do it on all platforms for simplicity.
- add darwin-aarch64 support to bluegl
- remove all aarch64 conditions in our cmakefiles, the architecture
is better handled in the source files (to allow fat binaries later)
