Hi @qnguyen,
Ardupilot is currently building Linux firmwares with gcc 7.5.0. You can specify a toolchain using the --toolchain argument for waf.
This is what it looks like this to use 9.2.1:
./waf configure --board=Navigator --toolchain=/opt/gcc-arm-9.2-2019.12-x86_64-arm-none-linux-gnueabihf/bin/arm-none-linux-gnueabihf