RFR: 8357913: Add `@Stable` to BigInteger and BigDecimal

[Shaojin Wen]

Some static final arrays of BigInteger and BigDecimal are stable and immutable. We should add `@Stable` to give the optimizer more information

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Commit messages:
 - BigInteger::mag add `@Stable`
 - add stable

Changes: https://git.openjdk.org/jdk/pull/25437/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25437&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357913
  Stats: 12 lines in 2 files changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/25437.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25437/head:pull/25437

PR: https://git.openjdk.org/jdk/pull/25437