ArmPkg/CpuDxe: Restore AArch64 system registers before returning from exception

ArmPkg/CpuDxe: Restore AArch64 system registers before returning from exception

Current EDK2 source code does actually trigger nested interrupted (even if
the PI spec says interrupt should not be nested).
This issue has highlighted the lack of restoring ELR_EL2/ELR_EL1 register.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off: Vijayakumar Subbu <vsubbu@nvidia.com>
Signed-off: Olivier Martin <olivier.martin@arm.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15481 6f19259b-4bc3-4df7-8a09-765794883524