setup.py picks the macOS-only -iwithsysroot/usr/include/ffi flag from
sys.platform, which describes the interpreter running the build rather than
the target, so cross-compiling from a macOS host hands an Apple clang flag to
the GNU cross compiler and the build fails. Use sysconfig.get_platform(),
which honours the _PYTHON_HOST_PLATFORM that python3-package.mk exports.