RFR: JDK-8325268: Add policy statement to langtools makefiles concerning warnings

[Joe Darcy]

Add policy statement about lint warnings to various langtools modules.

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

Commit messages:
 - JDK-8325268: Add policy statement to langtools makefiles concerning warnings

Changes: https://git.openjdk.org/jdk/pull/17718/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17718&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325268
  Stats: 14 lines in 3 files changed: 12 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17718.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17718/head:pull/17718

PR: https://git.openjdk.org/jdk/pull/17718