Add prefetch to xxhash func on gcc at least.

Improves hashing speed on Android by ~18%.
Add a NEON-optimized version of XXH32.

This takes at least 40% less time to hash on NEON/ARM devices.
Use defines on x86/SSE to avoid func pointers.

Unfortunately no such luck for Android.
Merge pull request #5720 from unknownbrackets/texcache

Optimize xxHash for ARM/NEON devices