rustbuild: check if compiler is final stage wrt the full bootstrap setting

src/bootstrap/lib.rs

@@ -914,4 +914,12 @@ fn new(stage: u32, host: &'a str) -> Compiler<'a> {
     fn is_snapshot(&self, build: &Build) -> bool {
         self.stage == 0 && self.host == build.config.build
     }
+
+    /// Returns if this compiler is to be treated as a final stage one, whether
+    /// we're performing a full bootstrap or not. Don't do that by comparing
+    /// stages with `2`!
+    fn is_final_stage(&self, build: &Build) -> bool {
+        let final_stage = if build.config.full_bootstrap { 2 } else { 1 };
+        self.stage >= final_stage
+    }
 }