MdeModulePkg/CapsuleRuntimeDxe: Add cache flush for IA32 and X64

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1462

The IA32 and X64 ARCH need cache flush function during capsule update.
And the cache flush is already implemented in arm ARCH, so add this
function CapsuleCacheWriteBack() to IA32 and X64 ARCH. And add a null
version for EBC.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Zhichao Gao <zhichao.gao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao Wu <hao.a.wu@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>