RFR: 8347754: [lw5] fix the order of nullness markers in signatures

[Vicente Romero]

Null markers must be at the end of signatures, currently they are being generated at the beginning see [1]

[1] https://bugs.openjdk.org/browse/JDK-8317766

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Commit messages:
 - 8347754: [lw5] fix the order of nullness markers in signatures

Changes: https://git.openjdk.org/valhalla/pull/1331/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1331&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347754
  Stats: 156 lines in 5 files changed: 71 ins; 25 del; 60 mod
  Patch: https://git.openjdk.org/valhalla/pull/1331.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1331/head:pull/1331

PR: https://git.openjdk.org/valhalla/pull/1331