Auto merge of #119035 - saethlin:remove-linker-requirement, r=onur-ozkan

Run Miri and mir-opt tests without a target linker

Normally, we need a linker for the target to build the standard library. That's only because `std` declares crate-type lib and dylib; building the dylib is what creates a need for the linker.

But for mir-opt tests (and for Miri) we do not need to build a `libstd.so`. So with this PR, when we build the standard library for mir-opt tests, instead of `cargo build` we run `cargo rustc --crate-type=lib` which overrides the configured crate types in `std`'s manifest.

I've also swapped in what seems to me a better hack than `BOOTSTRAP_SKIP_TARGET_SANITY` to prevent cross-interpreting with Miri from checking for a target linker and expanded it to mir-opt tests too. Whether it's actually better is up to a reviewer.

Author: bors