[foreign-jextract] RFR: 8252016: jextract should handle va_list

[Athijegannathan Sundararajan]

adding va_list support for jextract

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Commit messages:
 - 8252016: jextract should handle va_list

Changes: https://git.openjdk.java.net/panama-foreign/pull/285/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/285/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252016
  Stats: 131 lines in 5 files changed: 129 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/285.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/285/head:pull/285

PR: https://git.openjdk.java.net/panama-foreign/pull/285