[riscv-port] RFR: 8279826: riscv: Preserve result in native wrapper with +UseHeavyMonitors

[Yadong Wang]

Testing observed a few failures after JDK-8278387. On RISC-V, we followed JDK-8278489: The reason for the failures is in the native-wrappers, in the +UseHeavyMonitors paths, we don't preserve the result register after the native call.

Hotspot/jdk tier1 passed on the unmatched with -XX:+UseHeavyMonitors, and all jtregs were tested on Qemu without new failures.

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Commit messages:
 - 8279826: riscv: Preserve result in native wrapper with +UseHeavyMonitors

Changes: https://git.openjdk.java.net/riscv-port/pull/46/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=46&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279826
  Stats: 10 lines in 1 file changed: 6 ins; 4 del; 0 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/46.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/46/head:pull/46

PR: https://git.openjdk.java.net/riscv-port/pull/46