AVX2 Flash Attention (ggml-org#48)

* First version of AVX2 Flash attention

I simply took the Zen4 implementation and converted
platform specific stuff to methods of a struct providing
data loading/storing, conversions, multiply, add, etc.

Most likely not optimal as the Zen4 strategy has been
designed based on having 32 512-bit registers, so basically
we can have 4X more data stored in vector registers compared
to AVX2 with 16 x 256-bit.

It still gives a small speedup (~4% at 2048 tokens) for Gemma-2b.

* Fix Zenn4 parts broken via the AVX2 change

* Try smaller q_step - no improvement

* Fix ARM_NEON

I had forgotten to guard the AVX2/Zen4 implementation against __aarch64__

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Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>

Author: ikawrakow