RFR: 8318776: Require supports_cx8 to always be true [v7]

[David Holmes]

> As discussed in JBS all platforms (some tweaks to Zero are in progress) actually do support `cx8` i.e. 64-bit compare-and-exchange, so we can strip out the locked-based alternatives to using it and just add a guarantee that it is true at runtime. And all platforms except some ARM variants set `SUPPORTS_NATIVE_CX8`, so we can greatly simplify things. Summary of changes:
> - `_supports_cx8` field is only needed when `SUPPORTS_NATIVE_CX8` is not defined
> - Assertions for `supports_cx8()` are removed
> - Compiler predicates requiring `supports_cx8()` are removed
> - Access backend is greatly simplified without the need for lock-based alternative
> - `java.util.concurrent.AtomicLongFieldUpdater` is simplified without the need for a lock-based alternative
> 
> I did consider moving all the ARM `kuser_helper` related code to be only defined when `SUPPORTS_NATIVE_CX8` is not defined, but there was a theoretical risk this could change the behaviour if ARMv7 binaries were run on other ARM CPU's. I added a note to that effect in vm_version_linux_arm32.cpp so the ARM port maintainers could clean this up further if desired.
> 
> Testing:
> - All Oracle tiers 1-5 builds (which includes an ARMv7 build)
> - GHA builds/tests
> - Oracle tiers 1-3 sanity testing
> 
> Zero changes coming in via [JDK-8319777](https://bugs.openjdk.org/browse/JDK-8319777) will be merged when they arrive.
> 
> Thanks.

David Holmes has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains ten commits:

 - Merge
 - Fix typo
 - Merge with master and update Zero code accordingly
 - Merge branch 'master' into 8318776-supports_cx8
 - Remove unnecessary includes of vm_version.hpp.
   Fix copyright years.
 - Remove cx8 comment as no longer relevant (the spinlock is used regardless of cx8)
 - Remove suports_cx8() checks from gtest
 - Remove test for VMSupportsCX8
 - 8318776: Require supports_cx8 to always be true

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Changes: https://git.openjdk.org/jdk/pull/16625/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16625&range=06
  Stats: 460 lines in 39 files changed: 16 ins; 429 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/16625.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16625/head:pull/16625

PR: https://git.openjdk.org/jdk/pull/16625