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/CpuWorker.h

@@ -74,6 +74,7 @@ private:
     const bool m_yield;
     const CnHash::AlgoVariant m_av;
     const int m_astrobwtMaxSize;
+    const bool m_astrobwtAVX2;
     const Miner *m_miner;
     cryptonight_ctx *m_ctx[N];
     uint8_t m_hash[N * 32]{ 0 };