RFR: JDK-8286869: unify os::dir_is_empty coding across posix platforms

[Matthias Baesken]

The os::dir_is_empty coding is very similar across Posix platforms and could be unified to os_posix.cpp .

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Commit messages:
 - JDK-8286869

Changes: https://git.openjdk.java.net/jdk/pull/8750/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8750&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286869
  Stats: 76 lines in 4 files changed: 19 ins; 57 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8750.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8750/head:pull/8750

PR: https://git.openjdk.java.net/jdk/pull/8750