[15] RFR(XS): 8237479: 8230305 causes slowdebug build failure

[Severin Gehwolf]

Hi,

My push of hotspot's cgroup v2 support (JDK-8230305) broke the
slowdebug build. Sorry about this :-/ The issue is that
CgroupController and CgroupSubsystem are abstract classes and I missed
to declare virtual functions as pure.

This patch fixes that.

Bug: https://bugs.openjdk.java.net/browse/JDK-8237479
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8237479/01/webrev/

Testing: slowdebug/fastdebug/release builds work on Linux x86_64.
         Currently running through jdk/submit.

Thoughts?

Thanks,
Severin