[ONNX] Split into ONNX and ONNX_static recipes

[stemann]

Re-defined ONNX to be a recipe/package for shared library libonnx - with the static library in ONNX_static.

Also:

• Added windows support.
• Using static ProtocolBuffers library (for both dynamic and static libonnx).
• Reduced GCC version from 9 to 6 (note that protobuf is built with GCC 8).
• Added patch for more modern compilers (Clang/BSD systems and riscv64). This patch is already in upstream.