client VM build doesn't build in parallel anymore

[Matthias Klose]

Since b14 or b15, the client VM on x86 doesn't build anymore when building with

   --with-jvm-variants=client,server --with-num-cores=4

building with one to three cores seems to work however. The server and zero VMs 
build without issues and parallel builds.  Any idea which dependencies got dropped?

javac: file not found: 
/home/packages/openjdk/11/openjdk-11-11~18/build/buildtools/tools_classes/_the.BUILD_JFR_TOOLS_batch.tmp
make[4]: *** 
[/home/packages/openjdk/11/openjdk-11-11~18/build/buildtools/tools_classes/_the.BUILD_JFR_TOOLS_batch] 
Error 3
make[4]: *** Deleting file 
'/home/packages/openjdk/11/openjdk-11-11~18/build/buildtools/tools_classes/_the.BUILD_JFR_TOOLS_batch'
gensrc/GensrcJfr.gmk:40: recipe for target 
'/home/packages/openjdk/11/openjdk-11-11~18/build/buildtools/tools_classes/_the.BUILD_JFR_TOOLS_batch' 
failed
make[4]: Leaving directory '/home/packages/openjdk/11/openjdk-11-11~18/make/hotspot'
make[3]: *** [hotspot-client-gensrc] Error 2
make[3]: *** Waiting for unfinished jobs....
make/Main.gmk:249: recipe for target 'hotspot-client-gensrc' failed