Add a crt-debug target feature

[jdm]

This feature is designed to address #39016 but it has the curious property that it does not actually affect the behaviour of rustc's linking at all. Ultimately it's used to modify which CRT is linked by libc, much like the crt-static target feature.

[rust-highfive]

r? @eddyb

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[alexcrichton]

FWIW the crt-static feature went through an RFC. This PR seems pretty light on details and meat in general relative to that?

It would be good to at minimum include some tests which exercise this, for example ensuring that if C code is compiled on MSVC against the debug runtime that it works correctly when linked into Rust. This would likely also want to be added to documentation, probably around where crt-static is mentioned. I think we'll likely want to gate this to nightly to start out with as well since it's a new feature.

[totsteps]

ping from triage @eddyb any updates on the review?

Since alexcrichton left some comments, @jdm any opinions?

[jdm]

I'm going to close this until I've been able to verify that it actually addresses the use case that I care about.

[eddyb]

@gagan0723 Sorry, forgot to reassign.

r? @alexcrichton