Add webOS cross-compilation support, make openmp optional

[cscd98]

1. openmp is now optional for libretro as it was getting overridden
2. adds unix as a platform as this is what libretro-super sets
3. add cross compilation check for webOS. openMP is only supported on webOS 3.8.0, so I've added a check for the existence of openMP in the toolchain

[DerKoun]

Looks safe to merge (for someone like me, who doesn't know make files). I can't test it ATM, as the Github actions are outdated. But I'll probably just merge it, when I have some more time to look into it.

Out of curiosity, are there webOS devices powerful enough for bsnes-hd with at least some HD features? I know nothing about that ecosystem.

[cscd98]

Should be safe to merge as it shouldn't affect anything as the fallbacks did what it did before for the libretro bits.

As for powerfulness, I'm not really sure, that really depends on the TV model. Some users might be using a Cortex A9 in 2017 (so the base bsnes is more appropriate) whereas I'm using a Cortex A78 from 2024 model TV.

I suppose you could compare to mobile phones, different models with different CPU and GPUs.