Fix bug when reserve stack space to fill exception context.

Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Feng Tian <feng.tian@intel.com>












git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/UefiCpuPkg@15086 6f19259b-4bc3-4df7-8a09-765794883524