RFR: 8286540: Build failure caused by missing DefaultPollProvider implementation on AIX [v2]

[Tyler Steele]

> This change provides a stub implementation of DefaultPollerProvider on AIX. This is a temporary measure to ensure AIX can still be built while work on a full implementation is under way.

Tyler Steele has updated the pull request incrementally with two additional commits since the last revision:

 - Readily removes spurious space from pollset poller
 - Replace RuntimeException with UnsupportedOperationException

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8658/files
  - new: https://git.openjdk.java.net/jdk/pull/8658/files/6f8ac6d1..64d6e706

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8658&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8658&range=00-01

  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8658.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8658/head:pull/8658

PR: https://git.openjdk.java.net/jdk/pull/8658