RFR: Build: Update to JDK 15.0.2

[Andrea Cosentino]

Hello all,

Just a little PR for upgrading to JDK 15.0.2.

make images passed fine and make test too.

Thank you

-------------

Commit messages:
 - Build: Update to JDK 15.0.2

Changes: https://git.openjdk.java.net/skara/pull/989/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=989&range=00
  Stats: 14 lines in 3 files changed: 0 ins; 0 del; 14 mod
  Patch: https://git.openjdk.java.net/skara/pull/989.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/989/head:pull/989

PR: https://git.openjdk.java.net/skara/pull/989