Rollup merge of #151691 - Enselic:dylib-only-for-prefer-dynamic-v2, r=jdonszelmann

compiletest: Don't assume `aux-crate` becomes a `*.so` with `no-prefer-dynamic`

Since it does not make sense to do so. If someone prefers no dynamic stuff, the last thing they want to look for is an .so file. Also add a regression test. Without the fix, the test fails with:

    error: test compilation failed although it shouldn't!
    --- stderr -------------------------------
    error: extern location for no_prefer_dynamic_lib does not exist: .../auxiliary/libno_prefer_dynamic_lib.so
      --> .../no-prefer-dynamic-means-no-so.rs:9:5
       |
    LL |     no_prefer_dynamic_lib::return_42();
       |     ^^^^^^^^^^^^^^^^^^^^^

### Needed by:
-  rust-lang/rust#150591 because of rust-lang/rust#151271. But IMHO this PR makes sense on its own.

### Must wait for:
- [x] rust-lang/rust#151695

Author: Zalathar