We should test on CI that building the Engine with a custom toolchain and sysroot continue to work (that is, that the gn and ninja steps here https://medium.com/flutter/flutter-on-raspberry-pi-mostly-from-scratch-2824c5e7dcb1 continue to work). One way to do that might be to use the Linux clang toolchain we already pull and use it to build a libflutter_engine.so for armv7.