RFR: JDK-8289512: Fix GCC 12 warnings for adlc output_c.cpp

[Thomas Stuefe]

This fixes three warnings in my gcc 12 build on Ubuntu 22.04.

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Commit messages:
 - fix adlc gcc12 build

Changes: https://git.openjdk.org/jdk/pull/9335/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9335&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289512
  Stats: 13 lines in 1 file changed: 1 ins; 4 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/9335.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9335/head:pull/9335

PR: https://git.openjdk.org/jdk/pull/9335