UefiCpuPkg/SecCore: Add pre-memory AP vector

UefiCpuPkg/SecCore: Add pre-memory AP vector

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=2776

Add a vector at 0xFF000 (0xFFFFF000) that can be used by Init-SIPI-SIPI
to start an AP before memory is initialized. This vector jumps into the
same SEC entry point as the ordinary reset vector, with a special value
of "AP" in the DI register. The platform-specific SEC code is expected
to check for that value and take a different path for APs, if this
feature is supported by the platform.

Reviewed-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Deric Cole <deric.cole@intel.com>