RFR: 8269131: Update libxml2 to version 2.9.12

[Ambarish Rapte]

We currently use libxml2 version 2.9.10. It should be updated to latest stable release, which is version 2.9.12.
The steps to update libxml are documented in UPDATING.txt.

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Commit messages:
 - libxml update

Changes: https://git.openjdk.java.net/jfx/pull/562/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=562&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269131
  Stats: 5139 lines in 62 files changed: 1574 ins; 1362 del; 2203 mod
  Patch: https://git.openjdk.java.net/jfx/pull/562.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/562/head:pull/562

PR: https://git.openjdk.java.net/jfx/pull/562