RFR: 8332602: [s390x] Improve itable_stub

[Amit Kumar]

s390x Port similar to [JDK-8305959 (x86)](https://bugs.openjdk.org/browse/JDK-8305959) and [JDK-8307352(aarch64)](https://bugs.openjdk.org/browse/JDK-8307352)

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Commit messages:
 - z_r1 is free now, so we can use that
 - s390x Port

Changes: https://git.openjdk.org/jdk/pull/19698/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19698&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332602
  Stats: 158 lines in 3 files changed: 140 ins; 7 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/19698.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19698/head:pull/19698

PR: https://git.openjdk.org/jdk/pull/19698