RFR: 8345424: Move FindDebuginfoFiles out of FileUtils.gmk

[Magnus Ihse Bursie]

After [JDK-8339480](https://bugs.openjdk.org/browse/JDK-8339480), tier5 tests started failing in the `special:make:all` test. The culprit turned out to be that FindDebuginfoFiles was added to FileUtils.gmk, causing the find cache to be populated when it shouldn't be.

As a simple fix I am moving FindDebuginfoFiles into a separate file. The `DebugInfoUtils.gmk` is a bit odd since it is so specialized, and only shared among two files, but I could find no better way. The code is still a bit too complex to be duplicated, and I could not find any other place where it would belong better. The handling of debuginfo files is a bit spread out in the code base, so maybe this opens up for the chance to consolidate more debuginfo handling into this file further on.

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Commit messages:
 - 8345424: Move FindDebuginfoFiles out of FileUtils.gmk

Changes: https://git.openjdk.org/jdk/pull/22569/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22569&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345424
  Stats: 83 lines in 4 files changed: 60 ins; 23 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/22569.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22569/head:pull/22569

PR: https://git.openjdk.org/jdk/pull/22569