rustbuild: Fix uplifting stage1 on cross builds

When we pass `--host` the `self.hosts` array doesn't contain `self.build`, so
check `self.build` to see if we can uplift.

src/bootstrap/lib.rs

@@ -718,7 +718,7 @@ impl Build {
     fn force_use_stage1(&self, compiler: Compiler, target: Interned<String>) -> bool {
         !self.config.full_bootstrap &&
             compiler.stage >= 2 &&
-            self.hosts.iter().any(|h| *h == target)
+            (self.hosts.iter().any(|h| *h == target) || target == self.build)
     }
 
     /// Returns the directory that OpenSSL artifacts are compiled into if