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.c

@@ -0,0 +1,37 @@
+/** @file
+  Null instance of CPU Library.
+
+  Copyright (c) 2020, Intel Corporation. All rights reserved.<BR>
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+/**
+  Places the CPU in a sleep state until an interrupt is received.
+
+  Places the CPU in a sleep state until an interrupt is received. If interrupts
+  are disabled prior to calling this function, then the CPU will be placed in a
+  sleep state indefinitely.
+
+**/
+VOID
+EFIAPI
+CpuSleep (
+  VOID
+  )
+{
+}
+
+/**
+  Flushes all the Translation Lookaside Buffers(TLB) entries in a CPU.
+
+  Flushes all the Translation Lookaside Buffers(TLB) entries in a CPU.
+
+**/
+VOID
+EFIAPI
+CpuFlushTlb (
+  VOID
+  )
+{
+}