RFR: 8343122: RISC-V: C2: Small improvement for real runtime callouts

[Fei Yang]

Hi, please review this small improvement.

Currently, we do 11 instructions for real C2 runtime callouts (See riscv_enc_java_to_runtime).
Seems we can materialize the pointer faster with `movptr2`, which will reduce 2 instructions.
But we will need to reorder the original calling sequence a bit to make `t0` available for `movptr2`.

Testing on linux-riscv64 platform:
- [x] tier1-tier3 (release)
- [x] hotspot:tier1 (fastdebug)

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Commit messages:
 - 8343122: RISC-V: C2: Small improvement for real runtime callouts

Changes: https://git.openjdk.org/jdk/pull/21733/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21733&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343122
  Stats: 11 lines in 1 file changed: 4 ins; 2 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/21733.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21733/head:pull/21733

PR: https://git.openjdk.org/jdk/pull/21733