Add a warning about field_access_method_list not being portable

[JamesWrigley]

I may take a stab at fixing this later, but for now we can at least warn the users.

[JamesWrigley]

The failure on x86 nightly does not spark joy 👀 From looking at the testsets list I'm guessing it failed at the File test, but I don't see anything 32bit-specific that would fail. Could just be a spurious nightly failure given that the other x86 tests passed.

[Gnimuc]

It would be better to add warnings in the codegen code so people can not ignore them.

This is what we do to warn people about platform-specific definitions in system headers.

Clang.jl/src/generator/passes.jl

Line 95 in 8ed2dd2

show_info && @warn "[CollectDependentSystemNode]: found symbols in the system headers: $([n.id for (n,v) in x.dependents])"