RFR: 8340732: RISC-V: Refactor crc32 scalar version

[Hamlin Li]

Hi,
Can you help to review this patch?
As discussed in https://github.com/openjdk/jdk/pull/20910#discussion_r1755150447, it's helpful to refactor the existing scalar version of crc32 intrinsic.
Several refactoring are done in this pr,
1. Simplify the `len` usage, now it only decreases (i.e. change in one direction)
2. Simplify the code paths
3. Remove one instruction in `L_by4_loop`
4. Remove unnecessary code
5. Other misc

Thanks!

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/21150/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21150&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340732
  Stats: 57 lines in 2 files changed: 9 ins; 26 del; 22 mod
  Patch: https://git.openjdk.org/jdk/pull/21150.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21150/head:pull/21150

PR: https://git.openjdk.org/jdk/pull/21150