Rollup merge of #114166 - Enselic:libc-unavailable, r=Nilstrieb

Add regression test for resolving `--extern libc=test.rlib`

Closes #26043

I could not find a test for this particular use case. The closest I got was [`tests/ui/imports/issue-37887.rs`](https://github.com/rust-lang/rust/blob/master/tests/ui/imports/issue-37887.rs), but that is a regression test for a different use case (see rust-lang/rust#37887).