Build failure on aarch64 platform

[smlambert]

top.xml file checks for arm, not aarch. Need to add it as an os.arch property.

19:13:20 /home/jenkins/workspace/Grinder/openjdk-tests/systemtest/openj9-systemtest/openj9.build/include/top.xml:404: The following error occurred while executing this line:
19:13:20 /home/jenkins/workspace/Grinder/openjdk-tests/systemtest/openj9-systemtest/openj9.build/include/top.xml:518: java os.arch property value aarch64 not recognised. Update the get-platform-arch macrodef in openj9.build/include/top.xml