Avoid VPGATHER usage for most of Intel CPUs

* cipher/blake2.c (blake2b_init_ctx): Check for fast VPGATHER
for AVX512 implementation.
* src/hwf-x86.c (detect_x86_gnuc): Do not enable
HWF_INTEL_FAST_VPGATHER for Intel CPUs suffering from
"Downfall" vulnerability.

VPGATHER used to be fast on Intel CPU from Skylake to Tiger Lake,
but instruction is now very slow on these CPUs (slower than on
Haswell) because of mitigation introduced in new microcode version
for "Downfall" speculative execution vulnerability.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>