Rewrite search8.cc and search16.cc using intrinsics for portability

[torognes]

Registers xmm8-15 and r8-15 are unavailble on 32-bit platforms. Rewrite with intrinsics to avoid problem.

[torognes]

Or use https://github.com/RonnySoak/libssa

[torognes]

I do think we should spend time on support for 32-bit systems, they are too old.

[frederic-mahe]

Yes, I was checking https://store.steampowered.com/hwsurvey/directx/ and less than 2% of steam users are on a non-64bit system. If we assume swarm users follow the same trend, then very few people will actually use a 32-bit version of swarm. Besides, datasets larger than 4 GB (the product of one Illumina MiSeq run) are very frequent now.

[torognes]

Rewrite using intrinsics should perhaps be done anyway. Especially in order to port to other systems as described in issue #107. Could be based on the vsearch code.

[frederic-mahe]

Let's reopen the issue under a new title.

[torognes]

In the zobrist branch, the SIMD assembly code in search8.cc and search16.cc is now replaced by intrinsics for x86_64. Ready for porting to other architectures (#107).

[frederic-mahe]

Great! Let's close the issue then.