Fix HEXL benchmark performance degradation

[fboemer]

This PR fixes some performance degradation on the HEXL-enabled benchmarks, observed on ICX with clang-12. See below for one example, where this PR yields ~3.8x speedup on the N=8192 low-level NTT benchmarks.

To the best of my knowledge, using seal::UniformRandomGenerator turns out to be slow enough to cause some AVX512 transitions between benchmark iterations. AVX512 transitions also likely explains some of the strange runtime behavior described in #386. This PR switches to uniform_int_distribution for the benchmarks, which is fast enough to avoid the AVX512 transitions.

	Current SEAL	Current SEAL	uniform_int_dist	uniform_int_dist	Speedup	Speedup
BM	HEXL=OFF	HEXL=ON	HEXL=OFF	HEXL=ON	HEXL=OFF	HEXL=ON
n=8192 / log(q)=0 / UTIL / NTTForwardLowLevel	112	45.8	112	11.6	1.00	3.95
n=8192 / log(q)=0 / UTIL / NTTInverseLowLevel	83.4	46.1	83.1	12.3	1.00	3.75
n=8192 / log(q)=0 / UTIL / NTTForwardLowLevelLazy	69.1	43.5	68.9	11	1.00	3.95
n=8192 / log(q)=0 / UTIL / NTTInverseLowLevelLazy	81.6	45.4	81.2	12.1	1.00	3.75