arm-gcc version from ubuntu bionic repository is not supported in RIOT.
Both when building with `gnu` and `llvm`.

    arm-none-eabi-gcc --version
    arm-none-eabi-gcc (15:6.3.1+svn253039-1build1) 6.3.1 20170620

So detect this version and print an error if found.

The check is done on the building machine and not on the host when building in
docker.

The error can be disabled when building with WERROR=0.