StandaloneMmPkg: StandaloneMmCoreHobLib: Extend support for x64 Mm Core

StandaloneMmPkg: StandaloneMmCoreHobLib: Extend support for x64 Mm Core

This change adds support of x64 version of StandaloneMmCoreHobLib. It
brings in global variable "gHobList" through StandaloneMmCoreEntryPoint,
imports implementation from DxeCoreHobLib.inf to support x64 Mm Core and
moved shared functional plementations into a common file.

Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Supreeth Venkatesh <supreeth.venkatesh@arm.com>
Signed-off-by: Kun Qin <kun.q@outlook.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>