target-arm: Implement ARMv8 single-step handling for A64 code

Implement ARMv8 software single-step handling for A64 code:
correctly update the single-step state machine and generate
debug exceptions when stepping A64 code.

This patch has no behavioural change since MDSCR_EL1.SS can't
be set by the guest yet.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>