AVX2 optimized code for AstroBWT

Added "astrobwt-avx2" parameter in config.json, it's turned off ("false") by default.

4-5% speedup on CPUs with proper AVX2 support (AMD Ryzen starting with Zen2, Intel Core starting with Haswell).

There will be no speedup on the following CPUs:

- Intel Pentium/Celeron don't support AVX2
- AMD Zen/Zen+ have only half-speed AVX

GCC compiled version is faster without AVX2, MSVC compiled version is faster with AVX2

src/backend/cpu/CpuLaunchData.h

@@ -62,6 +62,7 @@ public:
     const bool hwAES;
     const bool yield;
     const int astrobwtMaxSize;
+    const bool astrobwtAVX2;
     const int priority;
     const int64_t affinity;
     const Miner *miner;