target-arm: aarch64: add support for stp (store pair)

This patch support basic GPR based store-pair instructions and includes
the generic do_gpr_st() function for store routines.

from Alexander Graf <agraf@suse.de> aarch64 series 11/60

---

v2 -> v3:

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

v3 -> v4
      - split from bigger patch (non vector)
      - better naming?