java os.name property value ${buildjdk.os.name} not recognised on build-osuosl-aix72-ppc64-3

Need to investigate

java os.name property value ${buildjdk.os.name} not recognised. Update the get-platform-prefix macrodef in stf.build/include/top.xml

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From https://ci.adoptium.net/job/Test_openjdk23_hs_extended.system_ppc64_aix_testList_0/5/console running on build-osuosl-aix72-ppc64-3 :

```
02:52:00 BUILD FAILED
02:52:00 /home/jenkins/workspace/Test_openjdk23_hs_extended.system_ppc64_aix_testList_0/aqa-tests/TKG/scripts/build_test.xml:95: The following error occurred while executing this line:
02:52:00 /home/jenkins/workspace/Test_openjdk23_hs_extended.system_ppc64_aix_testList_0/aqa-tests/system/common.xml:392: The following error occurred while executing this line:
02:52:00 /home/jenkins/workspace/Test_openjdk23_hs_extended.system_ppc64_aix_testList_0/aqa-tests/system/common.xml:383: The following error occurred while executing this line:
02:52:00 /home/jenkins/workspace/Test_openjdk23_hs_extended.system_ppc64_aix_testList_0/aqa-tests/system/STF/stf.build/include/top.xml:83: The following error occurred while executing this line:
02:52:00 /home/jenkins/workspace/Test_openjdk23_hs_extended.system_ppc64_aix_testList_0/aqa-tests/system/STF/stf.build/include/top.xml:517: The following error occurred while executing this line:
02:52:00 /home/jenkins/workspace/Test_openjdk23_hs_extended.system_ppc64_aix_testList_0/aqa-tests/system/STF/stf.build/include/top.xml:576: java os.name property value ${buildjdk.os.name} not recognised. Update the get-platform-prefix macrodef in stf.build/include/top.xml
02:52:00
02:52:00 Total time: 7 seconds
02:52:01 1
02:52:01 gmake[1]: *** [compile.mk:45: compile] Error 1
02:52:01 gmake[1]: Leaving directory '/home/jenkins/workspace/Test_openjdk23_hs_extended.system_ppc64_aix_testList_0/aqa-tests/TKG'
02:52:01 gmake: *** [makefile:87: compile] Error 2
[Pipeline] }
```

Start in stf.build/include/top.xml, especially the get-platform-prefix macrodef and the locations reported at lines 83, 517, and 576. Reproduce or inspect the AIX build failure to determine why ${buildjdk.os.name} is not recognized; done means the platform prefix is resolved without this error in the affected test build.

Written by the indexing model from the issue text.