[riscv-port-jdk11u:riscv-port] RFR: 8337780: RISC-V: C2: Change C calling convention for sp to NS

[Gui Cao]

Hi, Please help review this backport to riscv-port-jdk11u.
Clean backport of JDK-8337780 to reduce frame size by 16 bytes and improve performance for some C2 runtime stubs as we do not have to save sp on the method entry.

### Testing
- [x] Run tier1-3 tests on SOPHON SG2042 (release)

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Commit messages:
 - Backport 8337780: RISC-V: C2: Change C calling convention for sp to NS

Changes: https://git.openjdk.org/riscv-port-jdk11u/pull/26/files
  Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=26&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337780
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/riscv-port-jdk11u/pull/26.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk11u.git pull/26/head:pull/26

PR: https://git.openjdk.org/riscv-port-jdk11u/pull/26