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

[Aleksey Shipilev]

Testbug:
   https://bugs.openjdk.java.net/browse/JDK-8251949

On large TR 3970X, jtreg asks for MaxRAMFraction=0, which makes ZGC use 128M heap for some tests. 
These tests reliably fail with OOM in jdk-updates/jdk11u-dev:
   compiler/gcbarriers/UnsafeIntrinsicsTest.java
   compiler/loopopts/TestRangeCheckPredicatesControl.java

While they do not fail in jdk/jdk, I think it makes more sense to add explicit heap sizes in 
jdk/jdk, and then backport to 11u. The alternative is to backport ZGC improvements that make it more 
resilient at small heap sizes (like JDK-8234338?); but I am not feeling very comfortable doing and 
testing that.

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

Testing: affected tests in jdk/jdk and jdk-updates/jdk11u-dev

-- 
Thanks,
-Aleksey