let us compilation choice to be honored.
https://github.com/rust-lang/rust/issues/39900 

Index: src/bootstrap/src/lib.rs
--- src/bootstrap/src/lib.rs.orig
+++ src/bootstrap/src/lib.rs
@@ -1256,7 +1256,6 @@ impl Build {
         base.args()
             .iter()
             .map(|s| s.to_string_lossy().into_owned())
-            .filter(|s| !s.starts_with("-O") && !s.starts_with("/O"))
             .collect::<Vec<String>>()
     }
 
