UefiCpuPkg/CpuDxe: Guarantee GDT is below 4GB

UefiCpuPkg/CpuDxe: Guarantee GDT is below 4GB

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3233

GDT needs to be allocated below 4GB in 64bit environment
because AP needs it for entering to protected mode.
CPU running in big real mode cannot access above 4GB GDT.

But CpuDxe driver contains below code:
  gdt = AllocateRuntimePool (sizeof (GdtTemplate) + 8);
  .....
  gdtPtr.Base = (UINT32)(UINTN)(VOID*) gdt;

The AllocateRuntimePool() may allocate memory above 4GB.
Thus, we cannot use AllocateRuntimePool (), instead,
we should use AllocatePages() to make sure GDT is below 4GB space.

Signed-off-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>