Added a toolchain configuration to build for tvos

[SoylentGraham]

I needed a tvos build to go with ios, but the draco project doesn't seem to have one, so I've added one!

I'm a CMAKE newbie, so if anything is wrong, please correct it!

I use this exactly the same way I build ios (I need static libs); cmake ../ -DEMSCRIPTEN_GENERATE_BITCODE_STATIC_LIBRARIES=ON -DCMAKE_TOOLCHAIN_FILE=../cmake/toolchains/arm64-ios.cmake

But with this new toolchain cmake ../ -DEMSCRIPTEN_GENERATE_BITCODE_STATIC_LIBRARIES=ON -DCMAKE_TOOLCHAIN_FILE=../cmake/toolchains/arm64-tvos.cmake