sync x86_64-unknown-linux-gnu settings file with upstream

No need to carry the larger diff since upstream also moved the target to
use lld by default.

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

@@ -5,18 +5,6 @@ pub fn target() -> Target {
     base.cpu = "x86-64".into();
     base.plt_by_default = false;
     base.max_atomic_width = Some(64);
-    // Ferrocene elects to use lld as the linker on this target.
-    // Upstream still uses ld.
-    //
-    // Setting Lld::Yes causes `-fuse-ld=lld` to be added to the linker args
-    // (and by linker I mean `cc` as the linker as we use Cc::Yes)
-    base.linker_flavor = LinkerFlavor::Gnu(Cc::Yes, Lld::Yes);
-    // This setting causes `-B path/to/gcc-ld` to be added to the linker args.
-    // This means that the `ld.lld` wrapper for `rust-lld` appears in the
-    // path that `cc` uses to find `ld.lld` (also the path used for `cpp` and
-    // `cc1`, but we don't supply those so it goes back to the defaults for
-    // those tools).
-    base.link_self_contained = crate::spec::LinkSelfContainedDefault::with_linker();
     base.add_pre_link_args(LinkerFlavor::Gnu(Cc::Yes, Lld::No), &["-m64"]);
     base.stack_probes = StackProbeType::Inline;
     base.static_position_independent_executables = true;
@@ -31,6 +19,13 @@ pub fn target() -> Target {
     base.supports_xray = true;
     base.llvm_args = crate::spec::cvs!["--protect-from-escaped-allocas"];
 
+    // 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 {