[jdk20] RFR: 8298459: Fix msys2 linking and handling out of tree build directory for source zip creation [v2]

[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).

Christoph Langer has updated the pull request incrementally with one additional commit since the last revision:

  Use make conditionals instead of shell

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

Changes:
  - all: https://git.openjdk.org/jdk20/pull/9/files
  - new: https://git.openjdk.org/jdk20/pull/9/files/cff1a823..11b6dce8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk20&pr=9&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk20&pr=9&range=00-01

  Stats: 10 lines in 1 file changed: 0 ins; 0 del; 10 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