RFR: 8316186: RISC-V: Remove PlatformCmpxchg<4> [v3]

[Robbin Ehn]

> Hi, please consider.
> 
> I don't know the history behind the Cmpxhg<4>, but it is not needed anymore.
> Using compiler as in 8 byte case is just fine.
> 
> Passes hotspot-tier (qemu, vf2 still running), some manual extra testing via gtest, looked at compiler: output, e.g. https://godbolt.org/z/a31Gdqn8q
> 
> If you know the history please share, and if you have a reason why we should keep it please speak up!

Robbin Ehn has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge branch 'master' into 8316186-4cmpxchg
 - Remove redudant local
 - Removed

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15715/files
  - new: https://git.openjdk.org/jdk/pull/15715/files/6e5545aa..7a264fc0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15715&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15715&range=01-02

  Stats: 73725 lines in 1541 files changed: 28023 ins; 12708 del; 32994 mod
  Patch: https://git.openjdk.org/jdk/pull/15715.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15715/head:pull/15715

PR: https://git.openjdk.org/jdk/pull/15715