top-level/stage.nix: add static overlay

Adds the static overlay that can be used to build Nixpkgs statically.
Can be used like:

  nix build pkgsStatic.hello

Not all packages build, as some rely on dynamic linking.

pkgs/stdenv/darwin/portable-libsystem.sh

@@ -0,0 +1,10 @@
+# Make /nix/store/...-libSystem “portable” for static built binaries.
+# This just rewrites everything in $1/bin to use the
+# /usr/lib/libSystem.B.dylib that is provided on every macOS system.
+
+fixupOutputHooks+=('fixLibsystemRefs $prefix')
+
+fixLibsystemRefs() {
+  find "$1/bin" \
+    -exec install_name_tool -change @libsystem@ /usr/lib/libSystem.B.dylib {} \;
+}