Fix getrandom fallback test on platforms with panic=abort

[fs-rachel]

The recently-added linux-getrandom-fallback test fails on targets matching {aarch64*-none, armv7r-eabihf, thumbv7em-eabi*}. This is because those platforms set panic=abort by default, and the test relies on being able to catch a panic.

Mark the test as needs-unwind so that it won't be run in panic=abort configurations.

Also add extra debug information to the panic when getrandom() fails with an unexpected error code on Linux

[rustbot]

Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @Darksonn (or someone else) some time within the next two weeks.

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @ChrisDenton, libs
• @ChrisDenton, libs expanded to 13 candidates
• Random selection from 6 candidates

[joboet]

Oh, right... thanks!
@bors r+
r? joboet

[rust-bors]

📌 Commit 3886e7f has been approved by joboet

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 1. This pull request will be tested once the tree is reopened.

[joboet]

@bors r-
I hadn't realised CI hadn't even started yet... r=me with CI working.

@bors delegate+

[rust-bors]

This pull request was unapproved.

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[rust-bors]

✌️ @fs-rachel, you can now approve this pull request!

If @joboet told you to "r=me" after making some further change, then please make that change and post @bors r=joboet.

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