ptr_arg should ignore extern functions

[mrnossiom]

Fixes: #11181

changelog: [ptr_arg]: ignore extern functions that are not

I am not sure whether we should ignore other Rust calling conventions like rust-intrinsic, rust-call or rust-cold.

[rustbot]

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[Jarcho]

The call to check_mut_from_ref should happen before before the ABI check. mut_from_ref shouldn't depend on the function's ABI.

[Jarcho]

Thank you. @bors r+

[bors]

📌 Commit 7b8598d has been approved by Jarcho

It is now in the queue for this repository.

[bors]

⌛ Testing commit 7b8598d with merge a447725...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Jarcho
Pushing a447725 to master...