MdePkg DxeHobLib: Make GetHobList working before Constructor is called

MdePkg DxeHobLib: Make GetHobList working before Constructor is called

The latest PiSmmCore driver added several debug messages in the
function SmmAddMemoryRegion in Page.c. The function SmmAddMemoryRegion
is called by the library constructor
PiSmmCoreMemoryAllocationLibConstructor.

When PiSmmCoreMemoryAllocationLibConstructor is executed, the
constructor of DxeHobLib (HobLibConstructor in HobLib.c) is not
executed yet. But platform instance of DebugLib may need get hob
before printing any message. As a result, an ASSERT happens in the
function GetHobList.

The patch is to update GetHobList to get HOB list from system
configuration table when the HOB list is not retrieved and not cached
yet, and HobLibConstructor is also to be updated to just call
GetHobList.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Amy Chan <amy.chan@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>