stabilize -Clinker-features=-lld on x64 linux

This stabilizes a subset of the `-Clinker-features` components on x64 linux:
the lld opt-out.

The opt-in is not stabilized, as interactions with other stable flags require
more internal work, but are not needed for stabilizing using rust-lld by default.

Similarly, since we only switch to rust-lld on x64 linux, the opt-out is
only stabilized there. Other targets still require `-Zunstable-options`
to use it.

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -1379,7 +1379,7 @@ pub fn linker_and_flavor(sess: &Session) -> (PathBuf, LinkerFlavor) {
         }
     }
 
-    let features = sess.opts.unstable_opts.linker_features;
+    let features = sess.opts.cg.linker_features;
 
     // linker and linker flavor specified via command line have precedence over what the target
     // specification specifies