[lworld] RFR: Use stringStream to keep field layout output together.

[Coleen Phillimore]

Tested with -XX:+PrintFieldLayout comparing before and after.

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Commit messages:
 - Use stringStream to keep field layout output together.

Changes: https://git.openjdk.org/valhalla/pull/1132/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1132&range=00
  Stats: 18 lines in 1 file changed: 2 ins; 0 del; 16 mod
  Patch: https://git.openjdk.org/valhalla/pull/1132.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1132/head:pull/1132

PR: https://git.openjdk.org/valhalla/pull/1132