[jdk17u-dev] RFR: 8318915: Enhance checks in BigDecimal.toPlainString()

[Goetz Lindenmaier]

I backport this for parity with 17.0.16-oracle.

Fixed build.  AbstractStringBuilder.repeat(...) is not in 17.

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Commit messages:
 - Fix build
 - backport a6785e4d633908596ddb6de6d2eeab1c9ebdf2c3

Changes: https://git.openjdk.org/jdk17u-dev/pull/3510/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=3510&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318915
  Stats: 59 lines in 2 files changed: 43 ins; 2 del; 14 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/3510.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/3510/head:pull/3510

PR: https://git.openjdk.org/jdk17u-dev/pull/3510