UefiCpuPkg: Move MigrateGdt from DiscoverMemory to TempRamDone. (CVE-2019-11098)

UefiCpuPkg: Move MigrateGdt from DiscoverMemory to TempRamDone. (CVE-2019-11098)

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1614
REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3160

The GDT still in flash with commit 60b12e69fb1c8c7180fdda92f008248b9ec83db1
after TempRamDone

So move the action to TempRamDone event to avoid reading GDT from flash.

Signed-off-by: Guomin Jiang <guomin.jiang@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Cc: Debkumar De <debkumar.de@intel.com>
Cc: Harry Han <harry.han@intel.com>
Cc: Catharine West <catharine.west@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>