[11u] RFR (XS) 8251949: ZGC: Set explicit heap size for compiler/gcbarriers tests

[Aleksey Shipilev]

11u-specific bug:
   https://bugs.openjdk.java.net/browse/JDK-8251949

I suggested doing that in jdk/jdk initially, and then backport, but Per suggested it is best to do 
in 11u specifically.

11u webrev:
   https://cr.openjdk.java.net/~shade/8251949/webrev.11u.01/

Testing: affected tests

-- 
Thanks,
-Aleksey