bootstrap: support target specific config overrides

Author: weihanglo

Diff for: src/bootstrap/src/core/config/config.rs

@@ -644,7 +644,20 @@ impl Merge for TomlConfig {
         do_merge(&mut self.llvm, llvm, replace);
         do_merge(&mut self.rust, rust, replace);
         do_merge(&mut self.dist, dist, replace);
-        assert!(target.is_none(), "merging target-specific config is not currently supported");
+
+        match (self.target.as_mut(), target) {
+            (_, None) => {}
+            (None, Some(target)) => self.target = Some(target),
+            (Some(original_target), Some(new_target)) => {
+                for (triple, new) in new_target {
+                    if let Some(original) = original_target.get_mut(&triple) {
+                        original.merge(new, replace);
+                    } else {
+                        original_target.insert(triple, new);
+                    }
+                }
+            }
+        }
     }
 }