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[x86_64] Use 2-way GHASH pclmul update

Maamoun TK requested to merge mamonet/nettle:x86_ghash into master

I observed that pclmulqdq has latency of 7 cycles on Comet Lake arch and a reciprocal throughput of 7/7 = 1 so 2-way GHASH block update nearly doubles the performance speed on that architecture.

Tested on Intel Core i5-10300H

1-way (Former) 2-way
3014.85 Mbyte/s 6010.53 Mbyte/s

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