UefiCpuPkg: Simplify the implementation when separate exception stacks

UefiCpuPkg: Simplify the implementation when separate exception stacks

The API of InitializeSeparateExceptionStacks is just changed before, and
makes the struct CPU_EXCEPTION_INIT_DATA an internal definition.
Furthermore, we can even remove the struct to make core simpler.

Cc: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Signed-off-by: Zhiguang Liu <zhiguang.liu@intel.com>