[jdk8u-dev] RFR: 8209115: adjust libsplashscreen linux ppc64le builds for easier libpng update

[Martin Balao]

The 11u patch does not apply cleanly because of the file paths and because the context for the change in jdk/make/lib/Awt2dLibraries.gmk is slightly different.

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Commit messages:
 - Backport e4fdd0391733756f5b898371a66b38869d625c77

Changes: https://git.openjdk.org/jdk8u-dev/pull/371/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=371&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8209115
  Stats: 9 lines in 2 files changed: 6 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/371.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/371/head:pull/371

PR: https://git.openjdk.org/jdk8u-dev/pull/371