aarch32: Refactor inital entry

For historical reasons the early AArch32 code is structured differently
from the early AArch64 code, with some common code (including stack
setup) performed before we identify the mode we were entered in.

Align the structure of the early AArch32 code with that of the early
AArch64 code. This will make subsequent refactoring easier.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Cc: Akos Denke <akos.denke@arm.com>
Cc: Luca Fancellu <luca.fancellu@arm.com>