Fix passing of LLD threads flag

Author: Kobzol

src/bootstrap/lib.rs

@@ -1302,7 +1302,7 @@ impl Build {
         // If we use an external LLD, we don't know if it's new enough to support the required
         // threads flag. Therefore we invoke it to find it out.
         // The self-contained lld should always be new enough.
-        if test {
+        if test && matches!(self.config.lld_mode, LldMode::External) {
             let flag = self.lld_single_thread_flag(target);
             flags.push(format!("-Clink-arg=-Wl,{flag}"));
         }