Rollup merge of #133041 - madsmtm:print-deployment-target-env-var, r=davidtwco

Print name of env var in `--print=deployment-target`

The deployment target environment variable is OS-specific, and if you're in a place where you're asking `rustc` for the deployment target, you're likely to also wanna know the name of the environment variable. I myself wanted this for some code I'm working on in bootstrap, for example.

Behaviour before this PR:
```console
$ rustc --print=deployment-target --target=aarch64-apple-darwin
deployment_target=11.0
$ rustc --print=deployment-target --target=aarch64-apple-visionos
deployment_target=1.0
```

Behaviour after this PR:
```console
$ rustc --print=deployment-target --target=aarch64-apple-darwin
MACOSX_DEPLOYMENT_TARGET=11.0
$ rustc --print=deployment-target --target=aarch64-apple-visionos
XROS_DEPLOYMENT_TARGET=1.0
```

My _belief_ is that this option is extremely rarely used in general, and a GitHub search for "rustc print deployment-target" seems to confirm this, it revealed only the following actual pieces of code using this:
- https://github.com/PyO3/maturin/blob/b292ef69349f2a56cb8ab1b59fda0be3d3b9f138/src/build_context.rs#L1199-L1220
- https://github.com/rust-lang/cc-rs/blob/daab9244b03e244c4f2511944870d719c443f61f/src/lib.rs#L3422-L3426

`maturin` does `.split('=').last()`, so it will continue to work after this change, but `cc v1.0.84` did `.strip_prefix("deployment_target=")` since [this PR](https://github.com/rust-lang/cc-rs/pull/848), so it would break. That's _probably_ fine though, it was broken in a lot of scenarios anyway, and [got](https://github.com/rust-lang/cc-rs/pull/901) [reverted](https://github.com/rust-lang/cc-rs/pull/943) in `v1.0.85`.

So while this is _technically_ a breaking change, I really doubt that anyone is going to observe it, so it's probably fine.

``@BlackHoleFox`` wdyt?

``@rustbot`` label O-apple
r? compiler

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