RFR: 8349176: Speed up Integer/Long.toString via Unsafe.allocateUninitializedArray

[Shaojin Wen]

The byte[] allocated in Integer/Long.toString is fully filled, so we can use Unsafe.allocateUninitializedArray to create byte[] to improve performance.

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Commit messages:
 - simplify
 - use Unsafe.allocateUninitializedArray
 - revert StringConcatHelper newArray change
 - copyright
 - remove duplicate check
 - allocateUninitializedArray

Changes: https://git.openjdk.org/jdk/pull/23353/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23353&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349176
  Stats: 12 lines in 2 files changed: 6 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/23353.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23353/head:pull/23353

PR: https://git.openjdk.org/jdk/pull/23353