gcc_multi: Move $out/lib64/* to $lib/lib64

This prevents VirtualBox from having a copy of gcc in its closure.

pkgs/development/compilers/gcc/builder.sh

@@ -218,6 +218,15 @@ postInstall() {
         substituteInPlace "$i" --replace "$out" "$lib"
     done
 
+    if [ -n "$enableMultilib" ]; then
+        moveToOutput "lib64/lib*.so*" "$lib"
+        moveToOutput "lib64/lib*.la"  "$lib"
+
+        for i in "$lib"/lib64/*.{la,py}; do
+            substituteInPlace "$i" --replace "$out" "$lib"
+        done
+    fi
+
     # Remove `fixincl' to prevent a retained dependency on the
     # previous gcc.
     rm -rf $out/libexec/gcc/*/*/install-tools