target-arm: A64: 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>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
