Update to HEXL 1.2.0

Fixes #322 (closed)

Configuring using -DWITH_OPENMP=OFF -DCMAKE_CXX_COMPILER=clang++-13 -DCMAKE_C_COMPILER=clang-13 -DWITH_NATIVEOPT=ON (including the fix to clang-13 compilation here) and running on ICX yields the below benchmark results.

One major feature of HEXL v1.2 is a speedup on larger (~16384 coeffs) NTTs, which we see in the poly-hexl-benchmark-16k benchmark. HEXL isn't used to accelerate addition, so the changes in the addition benchmarks are noise.

numactl --physcpubind=0 ./bin/benchmark/lib-hexl-benchmark --benchmark_min_time=2

numactl --physcpubind=0 ./bin/benchmark/poly-hexl-benchmark-4k --benchmark_min_time=2

numactl --physcpubind=0 ./bin/benchmark/poly-hexl-benchmark-16k --benchmark_min_time=2