[11] RFR 8215637: client VM variant build error on zgc

[Boris Ulasevich]

Hi all,

Please review this fix for jdk-11.

JDK-11 client VM variant build fails because zgc (enabled by default 
now) depends on opto headers. In jdk-12 opto header #includes are 
disabled by #ifdef COMPILER, but I see no simple change to backport it. 
Here is the minimal change for jdk-11.

http://bugs.openjdk.java.net/browse/JDK-8215637
http://cr.openjdk.java.net/~bulasevich/8215637/webrev.01

thanks,
Boris