RFR: 8253478: (se) epoll Selector should use eventfd for wakeup instead of pipe

[Brian Burkhalter]

Please review this change which modifies the Linux `epoll(7)`-based `Selector` to use `eventfd(2)` instead of `pipe(2)` in its wakeup mechanism. The change passes all tier 1-tier 3 tests on Linux. Based on rudimentary testing, there does not appear to be any appreciable change in performance. No test is included as the code is covered well by existing tests.

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Commit messages:
 - 8253478: (se) epoll Selector should use eventfd for wakeup instead of pipe

Changes: https://git.openjdk.java.net/jdk/pull/2082/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2082&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253478
  Stats: 155 lines in 3 files changed: 142 ins; 4 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2082.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2082/head:pull/2082

PR: https://git.openjdk.java.net/jdk/pull/2082