Sandbox: use linux32 for x86-32 builds on x86-64 machines

  o _platform/linux.py: Add linux32 flag to send to sandbox bwrap when the
    build arch is x86-32 and the machines arch is x86-64 or similarly with
    aarch32 and aarch64.
  o sandbox/_sandboxbwrap.py: Use flag passed to start bwrap command with
    linux32 if set.