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/AArch64/SpeculationBarrier.S

@@ -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.
+#
+##------------------------------------------------------------------------------
+
+.text
+.p2align 2
+
+GCC_ASM_EXPORT(SpeculationBarrier)
+
+
+#/**
+#  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
+#  );
+#
+ASM_PFX(SpeculationBarrier):
+    dsb  sy
+    isb
+    ret