The test for avx512 could result in false-positives because the
_mm256_setr_epi64x intrinsic requires only AVX, not AVX512. Specifically what
is needed here is AVX512VL.
In addition, not passing the -mavx* args to the asm compilation failed the
build on macOS High Sierra and below (Apple LLVM version 10.0.0,
clang-1000.11.45.5)