RFR: 8269131: Update libxml2 to version 2.9.12 [v2]

[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.

Ambarish Rapte has updated the pull request incrementally with one additional commit since the last revision:

  changes as per review

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/562/files
  - new: https://git.openjdk.java.net/jfx/pull/562/files/4e1d6a43..2298581c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jfx&pr=562&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jfx&pr=562&range=00-01

  Stats: 1043 lines in 2 files changed: 10 ins; 2 del; 1031 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