MdePkg/BaseRngLib: Add support for ARMv8.5 RNG instructions

Make BaseRngLib more generic by moving x86-specific functionality into
'Rand' and adding files under 'AArch64' to support the optional ARMv8.5
RNG instruction RNDR that is a part of FEAT_RNG.

Signed-off-by: Rebecca Cran <rebecca@nuviainc.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>

MdePkg/Library/BaseRngLib/AArch64/ArmRng.asm

@@ -0,0 +1,39 @@
+;------------------------------------------------------------------------------
+;
+; ArmRndr() for AArch64
+;
+; Copyright (c) 2021, NUVIA Inc. All rights reserved.<BR>
+;
+; SPDX-License-Identifier: BSD-2-Clause-Patent
+;
+;------------------------------------------------------------------------------
+
+#include "BaseRngLibInternals.h"
+
+  EXPORT ArmRndr
+  AREA BaseLib_LowLevel, CODE, READONLY
+
+
+;/**
+;  Generates a random number using RNDR.
+;  Returns TRUE on success; FALSE on failure.
+;
+;  @param[out] Rand     Buffer pointer to store the 64-bit random value.
+;
+;  @retval TRUE         Random number generated successfully.
+;  @retval FALSE        Failed to generate the random number.
+;
+;**/
+;BOOLEAN
+;EFIAPI
+;ArmRndr (
+;  OUT UINT64 *Rand
+;  );
+;
+ArmRndr
+  mrs  x1, RNDR
+  str  x1, [x0]
+  cset x0, ne    // RNDR sets NZCV to 0b0100 on failure
+  ret
+
+  END