RFR: 7903190: jextract runtime helper issues

Mon May 16 14:07:42 UTC 2022

* using varargs flag during jextract time rather than at runtime
* avoiding redundant null checks in generated code
* avoiding String to MethodType conversion in RuntimeHelper.upcallStub method

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Commit messages:
 - 7903190: jextract runtime helper issues

Changes: https://git.openjdk.java.net/jextract/pull/33/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jextract&pr=33&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903190
  Stats: 49 lines in 4 files changed: 24 ins; 12 del; 13 mod
  Patch: https://git.openjdk.java.net/jextract/pull/33.diff
  Fetch: git fetch https://git.openjdk.java.net/jextract pull/33/head:pull/33

PR: https://git.openjdk.java.net/jextract/pull/33