Add explicit_extern_abis Feature and Enforce Explicit ABIs

[obeis]

Introduces the unstable explicit_extern_abis feature, requiring explicit ABIs in extern blocks. Hard errors are enforced with the feature enabled, and the 2027 edition prepares for full adoption.

RFC rust-lang/rfcs#3722

Update #134986

[rustbot]

r? @wesleywiser

rustbot has assigned @wesleywiser.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rustbot]

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

[ytmimi]

rustfmt changes look good 👍🏼

[compiler-errors]

Don't we need to mark style edition 2027 as unstable, @ytmimi?

[ytmimi]

You're totally right. I take back what I said 😅

[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

[ytmimi]

As @compiler-errors mentioned, we'd need to mark this as an unstable option in rustfmt.

Suggested change

	#[value = "2027"]
	#[doc_hint = "2027"]
	/// [Edition 2027]().
	Edition2027,
	#[value = "2027"]
	#[doc_hint = "2027"]
	#[unstable_variant]
	/// [Edition 2027]().
	Edition2027,

Though, If we're not moving forward with the edition implementation in this PR, then we can revert these rustfmt changes since we've already got an unstable 2027 style edition implemented in the upstream rustfmt

[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