mobile: there should be a test in CI which validates builds/functionality in 32 bits

Currently envoy-mobile breaks if anything in the core repo is not 32-bit compatible, but that doesn't run in CI, and is not easy for non-mobile devs to repro.

I think we should add a new test variant to the main CI which builds the core system (including core extensions, if that's the right term) and makes sure they pass tests.

This came up in the context of @kbaichoo 's PR #25149 .

I'm assigning this to 'mobile' as I don't think there are any non-mobile needs for 32-bit compiles but I think we should validate it outside the mobile toolset context.