Update SIMD builtins
Created by: melonedo
Add negation and rename pack builtins according to spec: https://github.com/openhwgroup/core-v-sw/blob/cb38ebc5f87269573799b26e3dc28da44488d5f6/specifications/corev-builtin-spec.md#listing-of-pulp-816-bit-simd-builtins-xcvsimd