return "", err
		}
		// Copy src/runtime/textflag.h for (at least) Test386EndToEnd in
		// cmd/asm/internal/asm.
		runtimePath := filepath.Join(dstbase, "src", "runtime")
		if err := os.MkdirAll(runtimePath, 0755); err != nil {
			return "", err
		}
		err = cp(
			filepath.Join(runtimePath, "textflag.h"),
			filepath.Join(cwd, "src", "runtime", "textflag.h"),
		)
		if err != nil {
			return "", err
		}
	}