target-arm: aarch64: add support for 'test and branch' imm

this patch adds emulation for the test and branch insns,
TBZ and TBNZ.

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

[claudio: adapted for new decoder,
          always compare with 0, remove a TCG temporary]

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

From Alexander Graf <agraf@suse.de> aarch64 series 40/60