Auto merge of #115987 - loongarch-rs:fix-transparent-union-abi, r=bjorn3

rustc_target/loongarch: Fix passing of transparent unions with only one non-ZST member

This ensures that `MaybeUninit<T>` has the same ABI as `T` when passed through an `extern "C"` function.

Fixes rust-lang/rust#115509

r? `@bjorn3`

Author: bors