Implement a cross compilation CI job from i386->x86_64

We should have at least one cross compilation nightly CI job from i386->x86_64; this will be useful in exposing the can of worms related to Int usages in our backend where Int is capped to 32-bit ranges and cause issue in 64-bit targets. #24893 (closed) is one recent example.

A debian i386 image can install crossbuild-essential-amd64 to bring in the required toolchain, and we don't even need a target wrapper for running the resulting code. The real heavy work would be triaging the testsuite and getting a reasonable subset running first, and I wouldn't be surprised there would be other breakages exposed.