Add and use the cpu_features library for feature detect

[unknownbrackets]

Unfortunately this "cross-platform C library" leaves some cross-platformness to be desired, so it's only used where supported. Still, it seems more well tested and has some fixes for misreported flags on ARM processors, etc.

I had to throw out its CMakeLists.txt, because it doesn't build at all on macOS and had relative path errors when used as an add_subdirectory().

It seems like it might at some point soon (?) support macOS/iOS and Windows for ARM64. It also does some logic to pull getauxval() dynamically because Android is annoying.

It would be worth trying this against some old Android like Xperia Play before merging.

Also threw in RISC-V half-float encodings, since they're ratified and just minor variation on existing instructions (not sure if we'd use them, maybe the convert between 16/32...)

-[Unknown]

[hrydgard]

Tried it on a reasonably old device (Shield Portable) and it's fine. Let's go for it, makes sense to use an existing, tested library for this.

[gchatelet]

@unknownbrackets

Unfortunately this "cross-platform C library" leaves some cross-platformness to be desired, so it's only used where supported. Still, it seems more well tested and has some fixes for misreported flags on ARM processors, etc.

Can you elaborate on which platforms are not supported enough? Actually it would be even better if you could open an issue so we can track it or +1 an existing issue so we can prioritize. Thx!

[unknownbrackets]

BSD (not just FreeBSD), UWP (Universal Windows Platform), iOS iirc, and Switch are the ones I remember. PPSSPP runs on a lot of devices and platforms, and cpu_features doesn't even run on Windows non-UWP ARM64 yet, so from the perspective of PPSSPP it is not all that cross-platform. It just supports a few of the most popular platforms.

-[Unknown]