docs(refs): Add unsafe to extern while using build scripts in Cargo Book

[namanlp]

What does this PR try to resolve?

Since rust 1.85.0, It is no longer sufficient to use the unsafe keyword only at the call site. It must also be applied to the extern statement, hence, Cargo Book must be updated

• closes Cargo reference build script example fails to compile #15289

and also update some more related content

How should we test and review this PR?

Run mdbook build. The unsafe extern should be seen at cargo/reference/build-script-examples.html#linking-to-system-libraries , cargo/reference/build-script-examples.html#building-a-native-library , cargo/reference/semver.html#repr-c-shuffle , cargo/reference/semver.html#repr-c-remove and cargo/reference/semver.html#repr-transparent-remove

Additional information

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[epage]

Thanks!

[epage]

I wonder if we should pull the example code out into a crate that we make a workspace member, like over at https://github.com/rust-cli/book