MdeModulePkg/DxeIplPeim: Support GHCB pages when creating page tables

MdeModulePkg/DxeIplPeim: Support GHCB pages when creating page tables

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198

GHCB pages must be mapped as shared pages, so modify the process of
creating identity mapped pagetable entries so that GHCB entries are
created without the encryption bit set. The GHCB range consists of
two pages per CPU, the first being the GHCB and the second being a
per-CPU variable page. Only the GHCB page is mapped as shared.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Acked-by: Hao A Wu <hao.a.wu@intel.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>