RFR: 8289258: ProblemList some failing custom loader tests with ZGC

[Calvin Cheung]

Problem listing the following tests in ProblemList-zgc.txt since they only failed with ZGC. See [JDK-8289257](https://bugs.openjdk.org/browse/JDK-8289257) for details.

- runtime/cds/appcds/customLoader/HelloCustom.java 
- runtime/cds/appcds/customLoader/HelloCustom_JFR.java 
- runtime/cds/appcds/dynamicArchive/HelloDynamicCustomUnload.java

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Commit messages:
 - 8289258: ProblemList some failing custom loader tests with ZGC

Changes: https://git.openjdk.org/jdk/pull/9301/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9301&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289258
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9301.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9301/head:pull/9301

PR: https://git.openjdk.org/jdk/pull/9301