Extend config to use metadata keys, like build script directives #13211
Description
In #11461, we were concerned about stepping on the toes of link metadata when adding new build script directives. #12201 solved this by locking down the old build script directive syntax and making the new syntax only accept metadata explicitly with the metadata key.
In doing this, we overlooked target.<triple>.<links>, see
cargo/src/cargo/util/config/target.rs
Lines 225 to 228 in 74ef21b