Rollup merge of #151353 - Enselic:aux-crate-opts, r=Zalathar

compiletest: Make `aux-crate` directive explicitly handle `--extern` modifiers

With `-Zunstable-options` it is possible to pass options to `--extern`. See here for an exhaustive list of possible options:
https://github.com/rust-lang/rust/blob/b5dd72d2921500c9d9e15f074e1d831adcaa3dee/compiler/rustc_session/src/config.rs#L2356-L2367

Using these options works with the `aux-crate` directive, but only because the options pretend to be part of the name. Make it clearer what `aux-crate` supports by explicitly handling `--extern` options.

This PR is step one of splitting up rust-lang/rust#151258 into smaller pieces.

r? @Zalathar

Author: matthiaskrgr