Every compiler toolchain consists of a side (frontend) that takes in the source code and a side that outputs the binary format for the target platform (backend). There's no reason why the backend couldn't work on any other platform than the one it's targeting. In the end, one merely transforms text files into sequences of bytes.
Cross-compiling in this fashion is an essential feature with MCU-oriented development, as compiling directly on those MCUs would be highly inefficient. There is, however, nothing magical about this process. In the case of GCC-based and GCC-compatible toolchains, one would still be interacting with the same interfaces on the toolchain, just with the tools usually prefixed with the target platform name to distinguish them from other toolchains for different targets. Essentially, instead of g++ one would use arm-none-eabi-g++
The resulting binaries would be in the format appropriate for that target platform.