[foreign-jextract] RFR: 8252016: jextract should handle va_list [v3]

[Athijegannathan Sundararajan]

> adding va_list support for jextract

Athijegannathan Sundararajan has updated the pull request incrementally with one additional commit since the last
revision:

  RuntimeHelper change was from a previous fix attempt. Removed it.
  Also no need for withName call for C_VA_LIST in OutputFactory.

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Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/285/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/285/files/4acd2292..6520f0aa

Webrevs:
 - full: https://webrevs.openjdk.java.net/panama-foreign/285/webrev.02
 - incr: https://webrevs.openjdk.java.net/panama-foreign/285/webrev.01-02

  Stats: 12 lines in 2 files changed: 0 ins; 10 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