Run cargo fix --edition on x86_64-unknown-linux-gnu

[eduardosm]

Description

This is the result of running cargo fix --edition on x86_64-unknown-linux-gnu, with some minor manual adjustments.

Cargo.toml still has edition 2015 since there are many other targets whose modules needs conversion.

Since ctest2 does not support crate:: in paths, a hack has been implemented in libc-test/build.rs, which makes a copy of src replacing every crate:: with ::.

cc #3898

Sources

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[rustbot]

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[bors]

☔ The latest upstream changes (presumably #4128) made this pull request unmergeable. Please resolve the merge conflicts.

[eduardosm]

  called `Result::unwrap()` on an `Err` value: Os { code: 30, kind: ReadOnlyFilesystem, message: "Read-only file system" }

Is the whole working copy read-only except target?

EDIT: It is.

libc/ci/run-docker.sh

Lines 50 to 51 in 01c7a63

	--volume "$(pwd)":/checkout:ro \
	--volume "$(pwd)"/target:/checkout/target \