UefiCpuPkg/CpuExceptionHandlerLib: fix incorrect init of exception stack

UefiCpuPkg/CpuExceptionHandlerLib: fix incorrect init of exception stack

This issue is introduced at following commit, which tried to add stack
switch support on behalf of Stack Guard feature.

    0ff5aa9cae1ea276668fa4398d047aa9fda3c2c7

The field KnownGoodStackTop in CPU_EXCEPTION_INIT_DATA is initialized to
the start address of array mNewStack. This is wrong. It must be the end
of mNewStack. This patch fixes this mistake.

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>