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.S

@@ -0,0 +1,37 @@
+#------------------------------------------------------------------------------
+#
+# ArmRndr() for AArch64
+#
+# Copyright (c) 2021, NUVIA Inc. All rights reserved.<BR>
+#
+# SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+#------------------------------------------------------------------------------
+
+#include "BaseRngLibInternals.h"
+
+.text
+.p2align 2
+GCC_ASM_EXPORT(ArmRndr)
+
+#/**
+#  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
+#  );
+#
+ASM_PFX(ArmRndr):
+  mrs  x1, RNDR
+  str  x1, [x0]
+  cset x0, ne    // RNDR sets NZCV to 0b0100 on failure
+  ret