RFR: 8317515: Unify the code of the parse*() families of methods in j.l.Integer and j.l.Long

[Raffaello Giulietti]

See the [JBS issue](https://bugs.openjdk.org/browse/JDK-8317515) for the details.

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Commit messages:
 - 8317515: Unify the code of the parse*() families of methods in j.l.Integer and j.l.Long

Changes: https://git.openjdk.org/jdk/pull/16050/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16050&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317515
  Stats: 458 lines in 2 files changed: 93 ins; 160 del; 205 mod
  Patch: https://git.openjdk.org/jdk/pull/16050.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16050/head:pull/16050

PR: https://git.openjdk.org/jdk/pull/16050