RFR: 8343020: SecureDirectoryStream not supported on MacOS

[David M. Lloyd]

OpenJDK will not produce SecureDirectoryStreams on MacOS. Support for SecureDirectoryStream on UNIX-like OSes is predicated on the `SUPPORTS_OPENAT` flag in UnixNativeDispatcher. That flag in turn is set when the runtime environment supports `openat`, `fstatat`, `unlinkat`, `renameat`, `futimesat`, and `fdopendir`.

This fails on MacOS because `futimesat` does not exist on that platform, apparently having been a proposed-but-not-accepted part of POSIX some time ago. While there is an indirect replacement that is supported on MacOS - `utimensat` - this is not actually needed, because the unique functionality provided by `futimesat` (that is, performing the action of `futimes` relative to an open directory file descriptor) is not utilized, since the only place this function is used passes `NULL` as the relative filename argument.

Replacing this with simply calling `futimes` instead allows `SecureDirectoryStream` to function on MacOS.

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Commit messages:
 - Simpler version that does not break arm32, hopefully
 - 8343020: SecureDirectoryStream not supported on MacOS

Changes: https://git.openjdk.org/jdk/pull/21696/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21696&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343020
  Stats: 14 lines in 2 files changed: 0 ins; 2 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/21696.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21696/head:pull/21696

PR: https://git.openjdk.org/jdk/pull/21696