Actually use self-contained lld in bootstrap when use-lld = "self-contained" is used

Before, we just used the global `lld` anyway.

Author: Kobzol

src/bootstrap/src/utils/helpers.rs

@@ -475,7 +475,20 @@ pub fn linker_flags(
 ) -> Vec<String> {
     let mut args = vec![];
     if !builder.is_lld_direct_linker(target) && builder.config.lld_mode.is_used() {
-        args.push(String::from("-Clink-arg=-fuse-ld=lld"));
+        match builder.config.lld_mode {
+            LldMode::External => {
+                args.push("-Clinker-flavor=gnu-lld-cc".to_string());
+                // FIXME(kobzol): remove this flag once MCP510 gets stabilized
+                args.push("-Zunstable-options".to_string());
+            }
+            LldMode::SelfContained => {
+                args.push("-Clinker-flavor=gnu-lld-cc".to_string());
+                args.push("-Clink-self-contained=+linker".to_string());
+                // FIXME(kobzol): remove this flag once MCP510 gets stabilized
+                args.push("-Zunstable-options".to_string());
+            }
+            LldMode::Unused => unreachable!(),
+        };
 
         if matches!(lld_threads, LldThreads::No) {
             args.push(format!(