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

[madsmtm]

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:

$ 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:

$ 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, so it would break. That's probably fine though, it was broken in a lot of scenarios anyway, and got reverted 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

[rustbot]

This PR modifies tests/run-make/. If this PR is trying to port a Makefile
run-make test to use rmake.rs, please update the
run-make port tracking issue
so we can track our progress. You can either modify the tracking issue
directly, or you can comment on the tracking issue and link this PR.

cc @jieyouxu

[madsmtm]

This also makes it easier to use in shell scripts, you can now do for example:

export $(rustc --print=deployment-target --target=$MY_APPLE_TARGET)
# Or
env $(rustc --print=deployment-target --target=$MY_APPLE_TARGET) clang --target $MY_APPLE_TARGET foo.c