UefiCpuPkg/UefiCpuPkg.dec: Add two new PCDs for stack switch

UefiCpuPkg/UefiCpuPkg.dec: Add two new PCDs for stack switch

Stack switch is required by Stack Guard feature. Following two PCDs are
introduced to simplify the resource allocation for initializing stack switch.

  gUefiCpuPkgTokenSpaceGuid.PcdCpuStackSwitchExceptionList
  gUefiCpuPkgTokenSpaceGuid.PcdCpuKnownGoodStackSize

PcdCpuStackSwitchExceptionList is used to specify which exception will
have separate stack for its handler. For Stack Guard feature, #PF must
be specified at least.

PcdCpuKnownGoodStackSize is used to specify the size of knwon good stack for an
exception handler. Cpu driver or other drivers should use this PCD to reserve
new stack memory for exceptions specified by above PCD.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Suggested-by: Ayellet Wolman <ayellet.wolman@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Jeff Fan <vanjeff_919@hotmail.com>
Reviewed-by: Jiewen.yao@intel.com