update AVX and AVX512 to support gcc < 10.1 and clang < 10

Steve Bronderrequested to merge
sbronder1/eigen:fix/old-gcc-avx into master

Description

Fixes #3021 (closed) with macros around the trouble intrinsics to allow a separate impl that is supported on the older compiler versions.

Reference issue

Fixes #3021 (closed)

Additional information

To be honest, I only have these old compiler versions on the Stan Jenkins CI. But locally I have tested this and it is passing. And with these patches the Stan Math repo is passing on gcc 8.

Would it be possible to make a 5.0.2? It looks like there have been a few nice bug fixes since 5.0.1. Dirk is also looking to update RcppEigen so it would be nice to have all these changes here instead of patching it downstream.

https://github.com/RcppCore/RcppEigen/issues/143

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