UefiCpuPkg/CpuExceptionHandlerLib: Code optimization to allow bigger stack

UefiCpuPkg/CpuExceptionHandlerLib: Code optimization to allow bigger stack

This commit is a code optimization to allow bigger seperate stack size in
ArchSetupExceptionStack. In previous code logic, CPU_STACK_ALIGNMENT bytes
will be wasted if StackTop is already CPU_STACK_ALIGNMENT aligned.

Signed-off-by: Dun Tan <dun.tan@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Reviewed-by: Abner Chang <abner.chang@amd.com>