GNUTOOLS-14765 Clean command errors

The "clean" stage removes all directories and run the "start" stage, at
which point the installdir is lost and the error rises:
this was introduced by refactoring here:
https://eu-gerrit-1.euhpc.arm.com/c/dsg-gnu/Internal/oban-scripts/+/566141/3/build-elf.sh#682

Implemented a generic mk_bin_dirs in utilities
Invoking the function in start stage so that in case of cleanup and
start, the directories are created.

Also fixed build-cross script which has a similar defect

Change-Id: Ifb9b0b15189c12dad2ba1d2e2843b84154953dff