RFR: 8331886: Allow markdown src file overrides

[Erik Joelsson]

For c, c++  and java source files, we have a built in system for letting more specific files override if there are multiple files with the same name found, by letting the first found file with a given name override any later found files with that name. This is typically used for OS specific variants or when needing to override a source file with a file from a custom source set. We would like to make it possible to do the same for markdown files used to generate man pages.

This will not have an immediate use i OpenJDK, but is needed for a custom override in proprietary code.

The change in Docs.gmk removes unnecessary extra loops so that SetupProcessMarkdown is called only once per module. This is necessary for the override mechanism to kick in for each module src set.

The logic in ProcessMarkdown.gmk is more or less copied from SetupNativeCompilation.

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Commit messages:
 - JDK-8331886

Changes: https://git.openjdk.org/jdk/pull/19132/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19132&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331886
  Stats: 77 lines in 2 files changed: 16 ins; 11 del; 50 mod
  Patch: https://git.openjdk.org/jdk/pull/19132.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19132/head:pull/19132

PR: https://git.openjdk.org/jdk/pull/19132