Add mgba recipe

This MR adds a recipe for mGBA, a Game Boy Advance emulator. Mostly works with a couple of issues.

Demo: https://www.youtube.com/watch?v=ppUsfzsSLcM

Known problems:

• Extreme stutter on input.
• Saving savegames might not work yet. Only loading them.
• Private mmaps seems to be borked on Redox. I added temporary workarounds for this.
• There's an issue with the SDL audio callbacks, caused by incorrect stack mapping. Temporary workarounds for now until it's fixed.
• The SDL/pte shutdown code started crashing recently. This affects every SDL app, not just mGBA.
• Sometimes crashes the kernel with "Grant should not exist" messages.
• Video corruption after multiple launches