RFR: 8316186: RISC-V: Remove PlatformCmpxchg<4> [v2]

[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 incrementally with one additional commit since the last revision:

  Remove redudant local

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/15715/files
  - new: https://git.openjdk.org/jdk/pull/15715/files/92782cb3..6e5545aa

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15715&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15715&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 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