Add required makefile for ARMv9.2-A.

#

# Copyright (C) 2023 The Android Open Source Project

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# Licensed under the Apache License, Version 2.0 (the "License");

# you may not use this file except in compliance with the License.

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#      http://www.apache.org/licenses/LICENSE-2.0

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# .mk file required to support build for the ARMv9.2-A arch variant.

# The file just needs to be present, it does not need to contain anything.