Add OpenMP SIMD pragma to function declarations in sleef.h (#404)

This patch adds vector ABI aliases to the vector functions.
It also adds "omp declare simd" pragma to the declaration of scalar functions in sleef.h.
This pragma is only enabled if SLEEF_ENABLE_OMP_SIMD is defined.
By the modifications above, the auto-vectorizer in gcc can convert calls to sleef scalar functions to the corresponding vector functions.