Add --target HOST alias for host target

[phil-opp]

This adds a HOST alias for the host target. It is useful when the default target is overridden in a .cargo/config file. It provides a platform-independent way to reset the target back to the host target. See #6775 for more motivation.

Closes #6775

[rust-highfive]

r? @dwijnand

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[dwijnand]

r? @alexcrichton

[alexcrichton]

Thanks for the PR and sorry for the delay!

I'm not personally convinced that we would want this feature, but others can of course voice their thoughts too! The semantics here aren't actually quite right where for various reasons --target $host is different than not passing --target at all. I also think that this is something that is pretty big (aliases for targets) and would need to be carefully considered in the context of everything that consumes targets in Rust (where everything takes opaque strings rather than being ready to handle aliases)

[phil-opp]

Thanks for the PR and sorry for the delay!

No worries, thanks for reviewing!

The semantics here aren't actually quite right where for various reasons --target $host is different than not passing --target at all. I also think that this is something that is pretty big (aliases for targets) and would need to be carefully considered in the context of everything that consumes targets in Rust (where everything takes opaque strings rather than being ready to handle aliases)

Ok, makes sense. It seemed like a small addition, but I now see that it is more complicated than that. I think there are better solutions to my problem (for example #6784), so I'm gonna close this PR and #6775 for now.