Explicitly specify symbol visibility when building std

This should be a no-op, since it's currently the default.

src/bootstrap/src/core/build_steps/compile.rs

@@ -266,6 +266,11 @@ impl Step for Std {
             cargo.rustflag(rustflag);
         }
 
+        // For now, we always build the standard library with interposable (default) visibility,
+        // since a build of the standard library with protected visibility would result in link
+        // errors if linked with GNU ld < 2.40.
+        cargo.rustflag("-Zdefault-visibility=interposable");
+
         let _guard = builder.msg(
             Kind::Build,
             compiler.stage,