JDK-8347909 causes build warnings on linux PPC64 which are treated as errors. I've used "-Winvalid-pch" to get better messages.

* For target hotspot_variant-server_libjvm_objs_sharedRuntimeTrans.o:
cc1plus: error: /jdk25_opt/hotspot/variant-server/libjvm/objs/precompiled/precompiled.hpp.gch: created and used with differing settings of '-mpower8-fusion-sign' [-Werror=invalid-pch]
cc1plus: error: one or more PCH files were found, but they were invalid
<command-line>: fatal error: precompiled.hpp: No such file or directory

* For target hotspot_variant-server_libjvm_objs_sharedRuntimeTrig.o:
cc1plus: error: /jdk25_opt/hotspot/variant-server/libjvm/objs/precompiled/precompiled.hpp.gch: created and used with differing settings of '-mpower8-fusion-sign' [-Werror=invalid-pch]
cc1plus: error: one or more PCH files were found, but they were invalid
<command-line>: fatal error: precompiled.hpp: No such file or directory

https://github.com/openjdk/jdk/blob/6a29a8110ec38b4adc8163ba8651cbc935353f1d/make/hotspot/lib/JvmOverrideFiles.gmk#L55
handles these 2 files. Seems like disabling PCH doesn't work any more.

We may need something like mentioned here:
https://github.com/openjdk/jdk/pull/23146#issuecomment-2595885031