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