fix(host-config): fix panic when cross-compiling with host-config

[arlosi]

What does this PR try to resolve?

When cross compiling (with --target), cargo panics with -Zhost-config.

This changes the explicit addition of the host target to only occur if the requested_kinds includes the host.

Otherwise, looking up the target_runner for the explicit_host_kind attempts to look up an entry in the target_config map that does not exist.

How to test and review this PR?

Validate that new test passes and does not panic.

Fixes #16664

[rustbot]

r? @weihanglo

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The reviewer was selected based on:

• Owners of files modified in this PR: @ehuss, @epage, @weihanglo
• @ehuss, @epage, @weihanglo expanded to ehuss, epage, weihanglo
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[arlosi]

Note that I'm not very familiar with this area of cargo, and there have been a chain of regressions and fixes here.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[weihanglo]

While this is nightly feature fix, it would be great if we can do a cargo submodule bump before beta branches off.

I am not available to do that this week though