RFR: 8331886: Allow markdown src file overrides [v2]

[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.

Erik Joelsson has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Remove debug printing
 - Merge branch 'master' into JDK-8331886-override-markdown
 - JDK-8331886

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/19132/files
  - new: https://git.openjdk.org/jdk/pull/19132/files/0bc0a668..17fee013

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19132&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19132&range=00-01

  Stats: 15 lines in 2 files changed: 11 ins; 2 del; 2 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