RFR: 8283328: Update libxml2 to 2.9.13

[Hima Bindu Meda]

We currently use libxml2 version 2.9.12. It should be updated to latest stable release, which is version 2.9.13.
The steps to update libxml are documented in UPDATING.txt.
There is compilation issue with the release 2.9.13, as mentioned here: https://gitlab.gnome.org/GNOME/libxml2/-/issues/349.
Updated the code with the changes as per the fix mentioned in above link.

Compiled and verified on Windows, Mac and Linux platforms.
Sanity testing looks fine.

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Commit messages:
 - Remove whitespace
 - Restore COPYING
 - Update formatting
 - Formatting changes
 - Windows : libxml update to 2.9.13
 - Update libxml to 2.9.13 for linux
 - Update libxml version to 2.9.13

Changes: https://git.openjdk.java.net/jfx/pull/764/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=764&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283328
  Stats: 55633 lines in 103 files changed: 5734 ins; 24999 del; 24900 mod
  Patch: https://git.openjdk.java.net/jfx/pull/764.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/764/head:pull/764

PR: https://git.openjdk.java.net/jfx/pull/764