Add explicit_extern_abis Feature and Enforce Explicit ABIs

[obeis]

The unstable explicit_extern_abis feature is introduced, requiring explicit ABIs in extern blocks. Hard errors will be enforced with this feature enabled in a future edition.

RFC rust-lang/rfcs#3722

Update #134986

[rustbot]

r? @wesleywiser

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[rustbot]

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

[traviscross]

@rustbot labels +S-blocked
@rustbot author

We're still finalizing the timing and name of the next edition, and in any case, I think I'd like to see a separate PR for adding the various bits for the next edition that's separate from any edition item.

[workingjubilee]

I do not think we will accept the edition changes being folded into this PR, @obeis

[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

[traviscross]

@rustbot author

[Nadrieril]

I second @traviscross's review comments, and r=me once they are addressed.

[obeis]

@bors r=nadrieril rollup

[bors]

@obeis: 🔑 Insufficient privileges: not in try users

[traviscross]

@bors r=traviscross,nadrieril rollup

[bors]

📌 Commit d17c04e has been approved by traviscross,nadrieril

It is now in the queue for this repository.