Rollup merge of #121392 - bjorn3:unify_dylib_loading, r=petrochenkov

Unify dylib loading between proc macros and codegen backends

As bonus this makes the errors when failing to load a proc macro more informative to match the backend loading errors. In addition it makes it slightly easier to patch rustc to work on platforms that don't support dynamic linking like wasm.

Author: matthiaskrgr