Plumbing to allow crosscompiles of Ubuntu to ARM variants

[pimvanpelt]

See discussion in https://github.com/cesanta/mongoose-os/issues/533

This PR is non-intrusive, as the default compiler toolkit chosen (x86_64-linux-gnu) is the one for AMD64:

lrwxrwxrwx 1 root root   5 May 20  2019 /usr/bin/gcc -> gcc-7
lrwxrwxrwx 1 root root   5 May 20  2019 /usr/bin/x86_64-linux-gnu-gcc -> gcc-7

Tested the change by compiling a few different applications for a few different targets:

for t in arm-linux-gnueabi arm-linux-gnueabihf aarch64-linux-gnu x86_64-linux-gnu; do
  mos build --clean --local --verbose --no-platform-check --build-var=COMPILER_TOOLKIT=$t
done

And they all compile cleanly -- although they are missing their *.a libraries.