RFR: 8307423: [s390x] Represent Registers as values [v4]

[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.

Amit Kumar has updated the pull request incrementally with one additional commit since the last revision:

  more suggestions from @RealLucy

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13805/files
  - new: https://git.openjdk.org/jdk/pull/13805/files/1552af91..514d7d86

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13805&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13805&range=02-03

  Stats: 10 lines in 1 file changed: 4 ins; 0 del; 6 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