Enable link-arg link kind inside of #[link] attribute

- Implement link-arg as an attribute
- Apply suggestions from review
  - Co-authored-by: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>
- Add unstable book entry

compiler/rustc_span/src/symbol.rs

@@ -934,6 +934,7 @@ symbols! {
         likely,
         line,
         link,
+        link_arg_attribute,
         link_args,
         link_cfg,
         link_llvm_intrinsics,