RFR: Update jextract 21 with latest fixes

[Maurizio Cimadamore]

This PR brings three changesets to the `master` branch, so that we can release updated binaries:

* https://git.openjdk.org/jextract/pull/132
* https://git.openjdk.org/jextract/pull/131
* https://github.com/openjdk/jextract/pull/137

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Commit messages:
 - 7903574: jextract should add lang=c to snippets containing C sources
 - 7903573: Jextract does not support atomic types
 - fix normalize method in RuntimeHelper.java.template

Changes: https://git.openjdk.org/jextract/pull/139/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=139&range=00
  Stats: 161 lines in 8 files changed: 158 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jextract/pull/139.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/139/head:pull/139

PR: https://git.openjdk.org/jextract/pull/139