RFR: 8331748: Update libxml2 to 2.12.6

[Hima Bindu Meda]

Updated libxml to v2.12.6. Verified build and sanity testing on all platforms. No issue seen

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Commit messages:
 - whitespace correction
 - update md file
 - Configure libxml on windows
 - configure libxml on linux
 - add missing headers
 - Update libxml to v2.12.6

Changes: https://git.openjdk.org/jfx/pull/1453/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1453&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331748
  Stats: 24912 lines in 115 files changed: 8485 ins; 11207 del; 5220 mod
  Patch: https://git.openjdk.org/jfx/pull/1453.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1453/head:pull/1453

PR: https://git.openjdk.org/jfx/pull/1453