RFR: 8340467: [mobile] duplicate symbols in libprefs.a on iOS

[Johan Vos]

Using both the unix native code and the mac native code results in 2 JNI_OnLoad functions in the native prefs library.
Don't use unix code when creating native lib for java.prefs on ios. We should only use the macos code.

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Commit messages:
 - Don't use unix code when creating native lib for java.prefs on ios.

Changes: https://git.openjdk.org/mobile/pull/29/files
  Webrev: https://webrevs.openjdk.org/?repo=mobile&pr=29&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340467
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/mobile/pull/29.diff
  Fetch: git fetch https://git.openjdk.org/mobile.git pull/29/head:pull/29

PR: https://git.openjdk.org/mobile/pull/29