java/openjdk21: fix build on powerpc64*

/wrkdirs/usr/ports/java/openjdk21/work/jdk21u-jdk-21.0.1-12-1/src/hotspot/share/runtime/javaThread.hpp:1117:10: fatal error: 'javaThread_bsd_ppc.hpp' file not found

^~~~~~~~~~~~~~~~~~~~~~~~~

/wrkdirs/usr/ports/java/openjdk21/work/jdk21u-jdk-21.0.1-12-1/src/hotspot/share/utilities/macros.hpp:618:40: note: expanded from macro 'OS_CPU_HEADER'

^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/wrkdirs/usr/ports/java/openjdk21/work/jdk21u-jdk-21.0.1-12-1/src/hotspot/share/utilities/macros.hpp:35:17: note: expanded from macro 'XSTR'

^~~~~~

/wrkdirs/usr/ports/java/openjdk21/work/jdk21u-jdk-21.0.1-12-1/src/hotspot/share/utilities/macros.hpp:32:17: note: expanded from macro 'STR'

^~

<scratch space>:74:1: note: expanded from here
"javaThread_bsd_ppc.hpp"
^~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.