enable rust-lld on x86_64-unknown-linux-gnu when requested

the `rust.lld` config enables rustc's `CFG_USE_SELF_CONTAINED_LINKER` env var, and we:
- set the linker-flavor to use lld
- enable the self-contained linker

this makes the target use the rust-lld linker by default

compiler/rustc_target/src/spec/mod.rs

@@ -612,6 +612,12 @@ impl LinkSelfContainedDefault {
             _ => "crt-objects-fallback",
         }
     }
+
+    /// Creates a `LinkSelfContainedDefault` enabling the self-contained linker for target specs
+    /// (the equivalent of `-Clink-self-contained=+linker` on the CLI).
+    pub fn with_linker() -> LinkSelfContainedDefault {
+        LinkSelfContainedDefault::WithComponents(LinkSelfContainedComponents::LINKER)
+    }
 }
 
 bitflags::bitflags! {