RFR: JDK-8312013: avoid UnixConstants.java.template warning: '__linux__' is not defined on AIX

[Matthias Baesken]

We run into this build warning on AIX :
/jdk/src/java.base/unix/classes/sun/nio/fs/UnixConstants.java.template:127:7: warning: '__linux__' is not defined, evaluates to 0 [-Wundef]
#elif __linux__
1 warning generated.

Looks like the preprocessor check should be adjusted, because the macro is not present on non-Linux systems.

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Commit messages:
 - JDK-8312013

Changes: https://git.openjdk.org/jdk/pull/14864/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14864&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312013
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14864.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14864/head:pull/14864

PR: https://git.openjdk.org/jdk/pull/14864