MdeModulePkg/DxeCore: Ensure FfsFileHeader 8 bytes aligned

MdeModulePkg/DxeCore: Ensure FfsFileHeader 8 bytes aligned

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=864
REF: CVE-2018-3630

To follow PI spec, ensure FfsFileHeader 8 bytes aligned.

For the integrity of FV(especially non-MemoryMapped FV) layout,
let CachedFv point to FV beginning, but not (FV + FV header).

And current code only handles (FwVolHeader->ExtHeaderOffset != 0) path,
update code to also handle (FwVolHeader->ExtHeaderOffset == 0) path.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>