Increase minimum clang version requirement for rsaz-avx512.pl
Increase minimum clang version requirement for rsaz-avx512.pl

The reason is that clang-6 does not enable proper -march flags by
default for assembly modules (rsaz-avx512.pl requires avx512ifma, avx512dq,
avx512vl, avx512f). This is not true for newer clang versions - clang-7 and
further work ok.

For older clang versions users who want to get optimization from this
file, we have a note in the OPENSSL_ia32cap.pod with the workaround that
proposes having a wrapper that forces using external assembler.

Fixes #14668: clang-6.0.0 build broken

Reviewed-by: Richard Levitte <5fb523282dd7956571c80524edc2dccfa0bd8234@openssl.org>
Reviewed-by: Matt Caswell <1fa2ef4755a9226cb9a0a4840bd89b158ac71391@openssl.org>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14671)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl