ARMv8 single-stepping requires the exception level that controls
the single-stepping to be in AArch64 execution state, but the
code being stepped may be in AArch64 or AArch32. Implement the
necessary support code for single-stepping AArch32 code.

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