ggml : add SSE3 and fp16 conversion lookup table (#368)

ggml : add SSE3 and fp16 conversion lookup table (#368)

* Improves WASM performance:
  On MacBook M1 Pro, I observe 25% faster using Firefox and 35% faster using Chrome

* Add support for SSE3 SIMD

* Add SSE3 to system information

* Add Imath support for fp16-fp32 conversions

* Add Imath to system information

* Wrap Imath calls to avoid static function warnings

* Drop Imath; Add lookup table for f16 -> f32 conversions

* Remove TODO comments

* Update SSE3 to new macro arguments

* Correct updated macro definitions

* Prefer static inline where possible

* ggml : static inlines + add public f16 <-> f32 conversions

Co-authored-by: Georgi Gerganov <redacted>