Rollup merge of rust-lang#77086 - ehuss:src-libunwind, r=Mark-Simulacrum

Include libunwind in the rust-src component.

Some targets, such as musl, need the libunwind source to build the unwind crate (referenced [here](https://github.com/rust-lang/rust/blob/0da58007451a154da2480160429e1604a1f5f0ec/library/unwind/build.rs#L142)).

Fixes rust-lang/wg-cargo-std-aware#59

src/bootstrap/dist.rs

@@ -1033,7 +1033,7 @@ impl Step for Src {
         copy_src_dirs(
             builder,
             &builder.src,
-            &["library"],
+            &["library", "src/llvm-project/libunwind"],
             &[
                 // not needed and contains symlinks which rustup currently
                 // chokes on when unpacking.