target-arm: Implement AArch64 EL1 exception handling

Implement exception handling for AArch64 EL1. Exceptions from AArch64 or
AArch32 EL0 are supported.

Signed-off-by: Rob Herring <rob.herring@linaro.org>
[PMM: fixed minor style nits; updated to match changes in
 previous patches; added some of the simpler cases of
 illegal-exception-return support]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
