RFR: 7511: Add 2021-12 target platform

[Marcus Hirt]

Not adding as default yet.

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Commit messages:
 - Add project file
 - 7511: Add 2021-12 target platform

Changes: https://git.openjdk.java.net/jmc/pull/360/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=360&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7511
  Stats: 49 lines in 5 files changed: 39 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jmc/pull/360.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/360/head:pull/360

PR: https://git.openjdk.java.net/jmc/pull/360