RFR: 8332602: [s390x] Improve itable_stub [v3]

[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)
> 
> Testing: I ran `tier1` test on fastdebug & release VM; I didn't see any regression there;

Amit Kumar has updated the pull request incrementally with three additional commits since the last revision:

 - Update src/hotspot/cpu/s390/macroAssembler_s390.cpp
 - Update src/hotspot/cpu/s390/macroAssembler_s390.cpp
 - polishing

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19698/files
  - new: https://git.openjdk.org/jdk/pull/19698/files/8692f7ad..19c3673c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19698&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19698&range=01-02

  Stats: 8 lines in 2 files changed: 2 ins; 4 del; 2 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