RFR: 8318776: Require supports_cx8 to always be true [v2]

[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 incrementally with one additional commit since the last revision:

  Remove test for VMSupportsCX8

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/16625/files
  - new: https://git.openjdk.org/jdk/pull/16625/files/3f2ec66f..b6dea4b6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16625&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16625&range=00-01

  Stats: 53 lines in 1 file changed: 0 ins; 53 del; 0 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