RFR: 8307423: s390x: Represent Registers as values

[Amit Kumar]

The recent Register implementation uses wild pointer (including null pointer) dereferences which exhibit undefined behavior. We should migrate away from pointer-based representation of Register values as it was done for x86 ([JDK-8292153](https://bugs.openjdk.org/browse/JDK-8292153)). Problems exist when trying to build with GCC 11.3.0 ([JDK-8307093](https://bugs.openjdk.org/browse/JDK-8307093)).

Tested `fastdebug, slowdebug, optimized, release build` , `tier1-test` on fastdebug build and build with GCC-9.5.0 as well.

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Commit messages:
 - s390x Port

Changes: https://git.openjdk.org/jdk/pull/13805/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13805&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307423
  Stats: 471 lines in 8 files changed: 76 ins; 221 del; 174 mod
  Patch: https://git.openjdk.org/jdk/pull/13805.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13805/head:pull/13805

PR: https://git.openjdk.org/jdk/pull/13805