[jfx11u] RFR: 8269131: Update libxml2 to version 2.9.12

[Kevin Rushforth]

Clean backport of third-party update to libxml2 2.9.12.

As an FYI, I tested this patch in my [test-kcr-11.0.13](https://github.com/kevinrushforth/jfx11u/tree/test-kcr-11.0.13) branch, which has the aggregate set of patches for the following bugs: [JDK-8211308](https://bugs.openjdk.java.net/browse/JDK-8211308), [JDK-8268915](https://bugs.openjdk.java.net/browse/JDK-8268915), [JDK-8269147](https://bugs.openjdk.java.net/browse/JDK-8269147), [JDK-8269131](https://bugs.openjdk.java.net/browse/JDK-8269131), [JDK-8268849](https://bugs.openjdk.java.net/browse/JDK-8268849), [JDK-8270479](https://bugs.openjdk.java.net/browse/JDK-8270479), [JDK-8272329](https://bugs.openjdk.java.net/browse/JDK-8272329).

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Commit messages:
 - 8269131: Update libxml2 to version 2.9.12

Changes: https://git.openjdk.java.net/jfx11u/pull/33/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx11u&pr=33&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269131
  Stats: 4134 lines in 62 files changed: 1582 ins; 1362 del; 1190 mod
  Patch: https://git.openjdk.java.net/jfx11u/pull/33.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx11u pull/33/head:pull/33

PR: https://git.openjdk.java.net/jfx11u/pull/33