poly1305/asm/poly1305-x86_64.pl: switch to pure AVX512F.

Convert AVX512F+VL+BW code path to pure AVX512F, so that it can be
executed even on Knights Landing. Trigger for modification was
observation that AVX512 code paths can negatively affect overall
Skylake-X system performance. Since we are likely to suppress
AVX512F capability flag [at least on Skylake-X], conversion serves
as kind of "investment protection".

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4758)