RFR: 8256499: Zero: enable Epsilon GC

[Aleksey Shipilev]

Following the [JDK-8255796](https://bugs.openjdk.java.net/browse/JDK-8255796) improvement that ditched the inline contiguous alloc use from Zero, we can now rely on GC interface to hook the GCs properly. Epsilon GC does not require anything else. Other GCs require a bit of fiddling and more testing (see e.g. [JDK-8256497](https://bugs.openjdk.java.net/browse/JDK-8256497)).

Additional testing:
 - [x] Linux x86_64 Zero {fastdebug,release}, `gc/epsilon` tests

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Commit messages:
 - 8256499: Zero: enable Epsilon GC

Changes: https://git.openjdk.java.net/jdk/pull/1269/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1269&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256499
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1269.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1269/head:pull/1269

PR: https://git.openjdk.java.net/jdk/pull/1269