If you are using Debian, like me, you have to enable multiarch to use 32-bit ADB and Fastboot and install their dependencies:

sudo dpkg --add-architecture i386
sudo apt-get update ; sudo apt-get install libc6:i386 libncurses5:i386 libstdc++6:i386

On most other systems you can run a 32 bit adb by installing the ia32lib package:

sudo apt-get install ia32libs