UefiCpuPkg/CpuDxe: Enable protection for newly added page table

One of the functionalities of CpuDxe is to update memory paging attributes.
If page table protection is applied, it must be disabled temporarily before
any attributes update and enabled again afterwards.

This patch makes use of the same way as DxeIpl to allocate page table memory
from reserved memory pool, which helps to reduce potential "split" operation
and recursive calling of SetMemorySpaceAttributes().

Laszlo (lersek@redhat.com) did a regression test on QEMU virtual platform with
one middle version of this series patch. The details can be found at

 https://lists.01.org/pipermail/edk2-devel/2017-December/018625.html

There're a few changes after his work.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

UefiCpuPkg/CpuDxe/CpuDxe.h

@@ -273,5 +273,7 @@ RefreshGcdMemoryAttributesFromPaging (
   VOID
   );
 
+extern BOOLEAN mIsAllocatingPageTable;
+
 #endif