MdePkg/BaseLib: implement SpeculationBarrier() for ARM and AArch64

Replace the dummy C implementation of SpeculationBarrier() with
implementations consisting of the recommended DSB SY + ISB sequence,
as recommended by ARM in the whitepaper "Cache Speculation Side-channels"
version 2.4, dated October 2018.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

MdePkg/Library/BaseLib/Arm/SpeculationBarrier.asm

@@ -0,0 +1,39 @@
+;------------------------------------------------------------------------------
+;
+; SpeculationBarrier() for AArch64
+;
+; Copyright (c) 2019, Linaro Ltd. All rights reserved.
+;
+; This program and the accompanying materials
+; are licensed and made available under the terms and conditions of the BSD License
+; which accompanies this distribution.  The full text of the license may be found at
+; http://opensource.org/licenses/bsd-license.php.
+;
+; THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+; WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+;
+;------------------------------------------------------------------------------
+
+    EXPORT SpeculationBarrier
+
+    AREA MemoryBarriers, CODE, READONLY
+
+;/**
+;  Uses as a barrier to stop speculative execution.
+;
+;  Ensures that no later instruction will execute speculatively, until all prior
+;  instructions have completed.
+;
+;**/
+;VOID
+;EFIAPI
+;SpeculationBarrier (
+;  VOID
+;  );
+;
+SpeculationBarrier
+    dsb
+    isb
+    bx    lr
+
+  END