RFR: 8367048: RISC-V: Correct pipeline descriptions of the architecture

[Dingli Zhang]

Hi,
Can you help to review this patch? Thanks!

This patch updates the RISC-V pipeline attributes to variable_size_instructions to properly account for the 2-byte compressed instructions from the C extension. 
Furthermore, it increases the max_instructions_per_bundle to 4 and adjusts the instruction_unit_size to match 4-issue RISC-V hardware like the UR-CP100.

### Test
- [x] Run tier1 and tier2 on sg2042

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Commit messages:
 - 8367048: RISC-V: Correct pipeline descriptions of the architecture

Changes: https://git.openjdk.org/jdk/pull/27134/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27134&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367048
  Stats: 12 lines in 1 file changed: 5 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/27134.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27134/head:pull/27134

PR: https://git.openjdk.org/jdk/pull/27134