Rollup merge of #147576 - Mark-Simulacrum:fix-offset-zst, r=nnethercote,RalfJung

Fix ICE on offsetted ZST pointer

I'm not sure this is the *right* fix, but it's simple enough and does roughly what I'd expect. Like with the previous optimization to codegen usize rather than a zero-sized static, there's no guarantee that we continue returning a particular value from the offsetting.

A grep for `const_usize.*align` found the same code copied to rustc_codegen_gcc and cranelift but a quick skim didn't find other cases of similar 'optimization'. That said, I'm not convinced I caught everything, it's not trivial to search for this.

Closes rust-lang/rust#147516

Author: matthiaskrgr