RFR: 8350137: After JDK-8348975, Linux builds contain man pages for windows only tools

[Nizar Benalla]

This patch fixes a regression where the windows tools man pages appeared on the Linux and the MacOS builds

The man pages found using `FindModuleManDirs` were used in both `Docs.gmk` used to generate the documentation for the JDK as well as in `LauncherCommon.gmk` here https://github.com/openjdk/jdk/blob/efbad00c4d7931177ccc5e9bce3b30dfbac94010/make/common/modules/LauncherCommon.gmk#L195

I've separated the logic between generated the docs and the man pages. I've checked on a MacOs machine and the man pages no longer appear.

This patch will need to be backported to JDK 24.

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Commit messages:
 - separate man page generation on UNIX systems and man page generation for JD

Changes: https://git.openjdk.org/jdk/pull/23697/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23697&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350137
  Stats: 8 lines in 2 files changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23697.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23697/head:pull/23697

PR: https://git.openjdk.org/jdk/pull/23697