[jdk20] RFR: 8298459: Fix msys2 linking and handling out of tree build directory for source zip creation

[Christoph Langer]

This PR fixes two issues.

In ZipSource.gmk we don't account for the possibility of the build directory or `$(SUPPORT_OUTPUTDIR)` not being in the source directory tree (`$(TOPDIR)`).
It doesn't manifest in a build error though since link-file-relative will then fall back to absolute linking. But it results in awkward paths.

Furthermore, there's a lurking issue in msys2 builds on Windows. There, ln would create deep copies of a file system tree instead of linking. If the file system to be linked contains long paths, it could result in 'File name too long' errors, like can be seen [here](https://github.com/gdams/jdk11u-dev/actions/runs/3563481453/jobs/5986319107). We can avoid this by calling Windows mklink to create a file system junction. Presumably that would also help build performance (though not verified).

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Commit messages:
 - Cleanups and remove trace statements
 - Fix linking in source zip creation

Changes: https://git.openjdk.org/jdk20/pull/9/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20&pr=9&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298459
  Stats: 17 lines in 2 files changed: 12 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk20/pull/9.diff
  Fetch: git fetch https://git.openjdk.org/jdk20 pull/9/head:pull/9

PR: https://git.openjdk.org/jdk20/pull/9