The current version of RetroArch (r12) does not support a forced orientation option. Instead it relies on the phone telling it what orientation it’s in, and RetroArch mimics it.
The problem of course is that if you have auto-rotate disabled on your phone (say because it’s annoying as shit) then RetroArch will be stuck in portrait mode, which is awful. Telling RetroArch to always display the game in landscape mode would solve this problem.
I’m sure it’d be easy to program this feature in, and I’d be grateful if it was