target-arm: aarch64: add support for movi

This patch adds emulation for the movi instruction. This includes the
various movn, movz and movk forms.

Signed-off-by: Alexander Graf <agraf@suse.de>

---

    v2 -> v3:

      - use extract32/sextract32 instead of get_bits and get_sbits
      - open code get_reg()

    v3 -> v4 (ajb):

       - applied RH's constant folding comment
       - move cpu_reg allocation to top