target-arm: aarch64: add support for EXTR

This patch adds emulation support for the EXTR instruction.

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

[claudio: adapted for new decoder, removed a few temporaries,
          fixed the 32bit bug, added checks for more
          unallocated cases]

Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>

From Alexander Graf <agraf@suse.de> aarch64 series 30/60