macOS - Create Universal Binaries (x64 and Arm64)

I managed to build universal binaries on macOS by modifying mac-toolchain.xml (manually adding the required `-arch` parameters for gcc, and unsetting `USE_PRECOMPILED_HEADERS`) and I can confirm that all of the object files and the executables are indeed universal. (I did not try this method with libraries, only executables)

It would be nice if HXCPP could do this out of the box.

*Note: this is not a bug report, but rather a feature request*

Thank you