target-arm: Handle the CPU being in AArch32 mode in the AArch64 set_pc

The AArch64 implementation of the set_pc method needs to be updated to
handle the possibility that the CPU is in AArch32 mode; otherwise there
are weird crashes when doing interprocessing in system emulation mode
when an interrupt occurs and we fail to resynchronize the 32-bit PC
with the TB we need to execute next.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
