The GCC versions of `objdump` and `nm` seem to have better out-of-the-box
support (for AArch64). Demangling works better, and instructions in objdump are
presented as 4 bytes instead of 4 * 1 bytes, which helps a lot.
Hence, switch to the GCC versions for now until LLVM has caught up.
20.04 comes with Ruby 2.7, which emits some annoying deprecation warnigs that
can't be silenced even when using the RUBYOPT variables suggested for doing so.
Work around that for now by reverting to Ubuntu 19.10 which still uses 2.5 and
does not have this bug.
This will require any non-standard use of this container to also
specify the rpi3.cfg path when invoking.
For example:
`docker run ... -f /openocd/tcl/interface/jlink.cfg -f /openocd/rpi3.cfg`
to work with JLink interface.
