tests: Build correct sandbox configuration on 32bit

Currently sandbox configuration defautls to 64bit and there is no
automation for building 32bit sandbox on 32bit hosts.

Use _LP64 macro as heuristic for detecting 64bit targets.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>