target-arm: aarch64: add support for compare and branch imm

this patch adds emulation for the compare and branch insns,
CBZ and CBNZ.

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

[claudio: adapted to new decoder,
          compare with immediate 0, use read_cpu_reg to get
          the 0 extension on (!sf)]

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

From Alexander Graf <agraf@suse.de> aarch64 series 33/60