target-arm: Add utility function for checking AA32/64 state of an EL

There are various situations where we need to behave differently
depending on whether a given exception level is in AArch64 or
AArch32 state. The state of the current exception level is stored
in env->aarch64, but there's no equivalent guest-visible architected
state bits for the status of the exception levels "above" the
current one which may still affect execution. At the moment we
only support EL1 (ie no EL2 or EL3) and insist that AArch64
capable CPUs run with EL1 in AArch64 state, but these may change
in the future, so abstract out the "what state is this?" check
into a utility function which can be enhanced later if necessary.

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