ggml : rewrite silu and softmax for cpu

This change upstreams llamafile's vectorized expf() functions. This lets
us compute softmax and silu more accurately than the short[65536] lookup
table that GGML previously used to make this operation go faster. We can
support aarch64 and sse2+ with the worst case rounding error of 2 ulp. I
wrote avx2 and avx512 implementations as well but they didn't offer much
advantage compared to sse2+fma to be worth the code complexity.

Author: jart