Add cross-platform builder in github action

[bensuperpc]

Cross-platform builder in github action:

• Dockcross : https://github.com/dockcross/dockcross
• Support over 10 CPU architectures (x86, x86_64, ARMv5, ARMv6, ARMv7, ARMv8, RISCV32, RISCV64...)
• Support 4 compilers (Clang, GCC and TinyCC, Emscripten...)
• Support 3 OS (Linux, Android, Windows) and WebAssembly
• Add dockcross + CMake (and Ninja) scripts
• Complete support C++17 and partial C++20 (GCC 11 and LLVG/Clang 12)
• CMake 3.21