scripts: update distclean

The toolchains are now built in .build/ rather than in targets/
so distclean has to get rid of that.

We also clean up the old dir, as users may have old samples that
still refer to the old location.

Do not hide messages during distclean, and do not ignore errors.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Compute Alpha-specific values

 CT_DoArchTupleValues () {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="${CT_ARCH}${CT_ARCH_ALPHA_VARIANT}"

 }