Update to HEXL version 1.2.1

[fboemer]

Update to HEXL v1.2.1, with speedup in the AVX512 NTT, particularly on N>=16384 coefficients. Also builds HEXL as a shared library iff SEAL is built as a shared library.

Compiling SEAL with clang-12 on ICX, running sealbench for 1000 iterations yields:

If SEAL is only merging PRs for bug fixes at the moment, feel free to keep this PR open

[WeiDaiWD]

From a quick glance, I see that there was a typo/bug in CMakeLists.txt. Thanks for catching it. It makes sense to release a patch just for this, and include all other PRs. I'll allocate time to handle both of your PRs.

[WeiDaiWD]

I see big improvement in Encode/Decode/MultPt, but small improvement or slowdown in other places, compared to HEXL 1.1.0. I tested it on a Cascade lake CPU. The observations is the same, with smaller improvement to MultPt. What would be a good justification to this new version in your opinion? It certainly helps encoding/decoding.

[fboemer]

Agreed, the biggest change/justification is a speedup in the AVX512 BFV Encode/Decode.

[WeiDaiWD]

Similar to the other PR, I have to apply your changes without leaving your commits in history. The commit message will be "Merge pull request #375 from fboemer/fboemer/hexl-v1.2.0" though.

I should have reverted contrib branch back to 3.6.6 after merging the bgv PR. Well, lessons learnt.