The march=cooperlake and march=sapphirerapids flags were never getting
added when building with Clang targetting those architectures. Instead
it was falling back to the skylake AVX512 implementation.
Clang added support for these two architectures in Clang 9 and Clang 12,
so introduce new checks for those versions to enable the appropriate
march flag, and fallback to skylake otherwise.
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