RFR: 8377185: [iOS] Native code compiler warnings and error building libprism_es2

[Jose Pereda]

This PR fixes the errors and some warnings that are thrown when building prism_es2 for iOS using clang.

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Commit messages:
 - Fix errors and warnings building prism_es2 for iOS

Changes: https://git.openjdk.org/jfx/pull/2064/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2064&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377185
  Stats: 16 lines in 5 files changed: 4 ins; 1 del; 11 mod
  Patch: https://git.openjdk.org/jfx/pull/2064.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2064/head:pull/2064

PR: https://git.openjdk.org/jfx/pull/2064