sdl: Fix null pointer dereference

See merge request redox-os/cookbook!204

userutils: Replace reference to x86_64-unknown-redox with $TARGET

See merge request !206

Sdl2 adjustments and sdl2 gears demo

See merge request redox-os/cookbook!207

Unbreaks builds for AArch64.

In some cases the SDL event loop could kick in before the window gets initialized.