libretro: Fix a crash from audio

[iyzsong]

Hello I find the libretro core could crash when produce the audio, SDL_LockAudioDevice seems fix it. Also I removed apply_variables in retro_run, since the only options can be changed durning runtime are the mouse ones, which I think is not very useful, remove it so code is more clear, and all options changes are required a restart to take effects.

Thanks.

[YuriSizuku]

Thank you for fixing this problem.