UefiCpuPkg/CpuExceptionHandlerLib: always clear descriptor data in advance

UefiCpuPkg/CpuExceptionHandlerLib: always clear descriptor data in advance

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1237

Sometimes the memory will be contaminated by random data left in last
boot (warm reset). The code should not assume the allocated memory is
always filled with zero. This patch add code to clear data structure
used for stack switch to prevent such problem from happening.

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