Document -Zlink-native-libraries

Originally added in rust-lang#70095.

Author: tmandry

src/doc/unstable-book/src/compiler-flags/link-native-libraries.md

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+# `link-native-libraries`
+
+This option allows ignoring libraries specified in `#[link]` attributes instead of passing them to the linker.
+This can be useful in build systems that manage native libraries themselves and pass them manually,
+e.g. with `-Clink-arg`.
+
+- `yes` - Pass native libraries to the linker. Default.
+- `no` - Don't pass native libraries to the linker.