Fix `-Ctarget-feature`s getting ignored after `crt-static`
The current behaviour introduced by commit a50a3b8e31 would discard any target features specified after `crt-static` (the only member of `RUSTC_SPECIFIC_FEATURES`). This is because it returned instead of continuing processing the next feature.
I wasn't entirely sure how the regression test should look like, but this one should do. If anyone has some suggestions, I'm happy to learn, it's my first test :)
I've confirmed that the test fails without the fix on `powerpc64le-unknown-linux-musl` and `x86_64-unknown-linux-gnu`.
cc ``@RalfJung``
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.