MdePkg/BaseCpuLibNull: Add Null version of CpuLib for host testing

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2798

The services in CpuLib usually generate exceptions in a unit test
host application.  Provide a Null instance that can be safely used.

This Null instance can also be used as a template for implementing
new instances of CpuLib.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Library/BaseCpuLibNull/BaseCpuLibNull.inf

@@ -0,0 +1,26 @@
+## @file
+#  Null instance of CPU Library.
+#
+#  Copyright (c) 2020, Intel Corporation. All rights reserved.<BR>
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
+[Defines]
+  INF_VERSION                    = 0x00010005
+  BASE_NAME                      = BaseCpuLibNull
+  MODULE_UNI_FILE                = BaseCpuLibNull.uni
+  FILE_GUID                      = 8A29AAA5-0FB7-44CC-8709-1344FE89B878
+  MODULE_TYPE                    = BASE
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = CpuLib
+
+#
+#  VALID_ARCHITECTURES           = IA32 X64 EBC ARM AARCH64 RISCV64
+#
+
+[Sources]
+  BaseCpuLibNull.c
+
+[Packages]
+  MdePkg/MdePkg.dec