JDK-8286562 added the `-Wstringop-overflow` pragma, which cannot be understood by GCC 6. There are already ways to make such pragmas conditional on GCC version.

```
=== Output from failing command(s) repeated here ===
* For target hotspot_variant-server_libjvm_objs_ageTableTracer.o:
In file included from /home/buildbot/worker/build-jdkX-debian9/build/src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdLoadBarrier.inline.hpp:33:0,
                 from /home/buildbot/worker/build-jdkX-debian9/build/src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceId.inline.hpp:30,
                 from /home/buildbot/worker/build-jdkX-debian9/build/src/hotspot/share/jfr/writers/jfrWriterHost.inline.hpp:31,
                 from /home/buildbot/worker/build-jdkX-debian9/build/src/hotspot/share/jfr/writers/jfrEventWriterHost.hpp:28,
                 from /home/buildbot/worker/build-jdkX-debian9/build/src/hotspot/share/jfr/writers/jfrEventWriterHost.inline.hpp:28,
                 from /home/buildbot/worker/build-jdkX-debian9/build/src/hotspot/share/jfr/writers/jfrNativeEventWriter.hpp:30,
                 from /home/buildbot/worker/build-jdkX-debian9/build/src/hotspot/share/jfr/recorder/service/jfrEvent.hpp:33,
                 from /home/buildbot/worker/build-jdkX-debian9/build/build/linux-x86_64-server-fastdebug/hotspot/variant-server/gensrc/jfrfiles/jfrEventClasses.hpp:12,
                 from /home/buildbot/worker/build-jdkX-debian9/build/src/hotspot/share/jfr/jfrEvents.hpp:32,
                 from /home/buildbot/worker/build-jdkX-debian9/build/src/hotspot/share/gc/shared/ageTableTracer.cpp:28:
/home/buildbot/worker/build-jdkX-debian9/build/src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdBits.inline.hpp: In static member function 'static void JfrTraceIdBits::store(jbyte, const T*)':
/home/buildbot/worker/build-jdkX-debian9/build/src/hotspot/share/jfr/recorder/checkpoint/types/traceid/jfrTraceIdBits.inline.hpp:141:51: error: unknown option after '#pragma GCC diagnostic' kind [-Werror=pragmas]
   PRAGMA_DISABLE_GCC_WARNING("-Wstringop-overflow")
```