crypto/x86_64cpuid.pl: suppress AVX512F flag on Skylake-X.

It was observed that AVX512 code paths can negatively affect overall
Skylake-X system performance. But we are talking specifically about
512-bit code, while AVX512VL, 256-bit variant of AVX512F instructions,
is supposed to fly as smooth as AVX2. Which is why it remains unmasked.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4838)