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! {

compiler/rustc_target/src/spec/targets/x86_64_unknown_linux_gnu.rs

@@ -18,6 +18,13 @@ pub fn target() -> Target {
         | SanitizerSet::THREAD;
     base.supports_xray = true;
 
+    // When we're asked to use the `rust-lld` linker by default, set the appropriate lld-using
+    // linker flavor, and self-contained linker component.
+    if option_env!("CFG_USE_SELF_CONTAINED_LINKER").is_some() {
+        base.linker_flavor = LinkerFlavor::Gnu(Cc::Yes, Lld::Yes);
+        base.link_self_contained = crate::spec::LinkSelfContainedDefault::with_linker();
+    }
+
     Target {
         llvm_target: "x86_64-unknown-linux-gnu".into(),
         metadata: crate::spec::TargetMetadata {