target-arm: A64: Add last AdvSIMD Integer to FP ops

This adds the remaining [US]CVTF operations to both C3.6.14 and C3.6.17
groups of opcodes.

---

QUESTIONS:
  - Should we factor out the common code from handle_fpfpcvt?
  - Can the clear_vec_high be pushed into common code?
  - Can the element calculation be pushed into common code?