ggml : move FP16 <-> FP32 code to ggml-impl.h (#3861)
* ggml : move FP16 <-> FP32 stuff to ggml-impl.h
ggml-ci
* tests : fix ARM build
* ggml : explicitly initialize deprecated type traits
* ggml : add math.h to ggml-impl.h
* ggml : remove duplicate static assert macros
* ggml : prefix lookup tables with ggml_
ggml-ci
* ggml-impl : move extern "C" to start of file
