Auto merge of #41757 - alexcrichton:stabilize-crt-static, r=japaric

Auto merge of #41757 - alexcrichton:stabilize-crt-static, r=japaric

rustc: Stabilize `-C target-feature=+crt-static`

This commit stabilizes the `crt-static` feature accepted by the compiler. Note
that this does not stabilize the `#[cfg]` attribute for `crt-static` as
that's going to be covered by #29717. This only stabilizes a few small pieces:

* The `crt-static` feature as accepted by the `-C target-feature` flag, and its
  connection with the platform-specific definition of `crt-static`.
* The semantics of `--print cfg` printing out activated `crt-static` feature, if
  available.

This should be enough to get the benefits of `crt-static` on stable Rust with
MSVC and with musl, but sidsteps the issue of stabilizing #29717 first.

Closes #37406