Document that we only support building with Cerbero on 64-bit Windows

Original title: MinGW multilib toolchain is built for running only on 64-bit Windows

This means you can't use Cerbero on 32-bit Windows. Note that building for 32-bit still works fine as long as your Windows is 64-bit. @ylatuya was there a reason for this that we forgot to document or was this just an oversight? This is a regression in master over 1.16, we should probably fix it by shipping a toolchain that can run on 32-bit Windows (which will also work on 64-bit thanks to WOW64).