target/arm: Implement FEAT_HBC

FEAT_HBC (Hinted conditional branches) provides a new instruction
BC.cond, which behaves exactly like the existing B.cond except
that it provides a hint to the branch predictor about the
likely behaviour of the branch.

Since QEMU does not implement branch prediction, we can treat
this identically to B.cond.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>