cpuminer-opt-3.20.1

sph_blake2b optimized 1-way SSSE3 & AVX2.
Removed duplicate Blake2b used by Power2b algo, will now use optimized sph_blake2b.
Removed imprecise hash & target display from rejected share log.
Share and target difficulty is now displayed only for low diificulty shares.
Updated configure.ac to check for AVX512 asm support.
Small optimization to Lyra2 SSE2.