Enable cfg predicate for target_feature = "crt-static" only if the target supports it

[petrochenkov]

That's what all other target_features do.

[rust-highfive]

r? @matthewjasper

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[petrochenkov]

The annoying question here is "what happens if the target supports crt-static for executables, but not for libraries" (or vice versa).

At configuration time we don't know the crate type(s) produced by the code - it may be specified by attributes.
We also may produce multiple crate types in one run of rustc, including both executables and libraries, and code for them must not diverge until link time.
So we set the cfg predicate if the target supports crt-static for any of the possible crate types, basically.

Fortunately, cfg(target_feature = "crt-static") is generally used in lazy link-time configuration (#[link(name = "foo", cfg(target_feature = "crt-static"))]) rather than in early expansion-time configuration, so it shouldn't be a problem.

[bors]

☔ The latest upstream changes (presumably #71807) made this pull request unmergeable. Please resolve the merge conflicts.

[matthewjasper]

@bors r+

[bors]

📌 Commit 33b6631 has been approved by matthewjasper

[bors]

⌛ Testing commit 33b6631 with merge 8d16eeb...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: matthewjasper
Pushing 8d16eeb to master...