target/arm: Create helper_exception_swstep

target/arm: Create helper_exception_swstep

Move the computation from gen_swstep_exception into a helper.

This fixes a bug when:
  - MDSCR_EL1.KDE == 1 to enable debug exceptions within EL_D itself
  - we singlestep an ERET from EL_D to some lower EL

Previously we were computing 'same el' based on the EL which
executed the ERET instruction, whereas it ought to be computed
based on the EL to which ERET returned.  This happens naturally
with the new helper, which runs after EL has been changed.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220609202901.1177572-14-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>