target-arm: aarch64: add support for conditional branches

This patch adds emulation for the conditional branch (b.cond) instruction.

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

[claudio: adapted to new decoder structure,
          reused arm infrastructure for checking the flags]

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

From Alexander Graf <agraf@suse.de> aarch64 series 34/60
target-arm: aarch64: add support for conditional select

this patch adds support for "C3.5.6 Conditional select",
which means CSEL, CSINC, CSINV, CSNEG.

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

From Alexander Graf <agraf@suse.de> aarch64 series 43/60