RFR: 8316182: RISC-V: SA stack walking code having trouble finding sender frame when invoking LambdaForms is involved

[Feilong Jiang]

Hi, please consider.
Inspired by [JDK-8313800](https://bugs.openjdk.org/browse/JDK-8313800). RISC-V also treats x8/fp as a callee-saved scratch register for some time, and it is freely used by C2-generated code. Therefore, any code in SA that uses getFP() in a compiled frame is very likely to come to grief.

Testing:
- [x] hotspot_serviceability
- [x] jdk_svc
- [x] tier1

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Commit messages:
 - riscv fix

Changes: https://git.openjdk.org/jdk/pull/15714/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15714&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316182
  Stats: 14 lines in 1 file changed: 0 ins; 7 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/15714.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15714/head:pull/15714

PR: https://git.openjdk.org/jdk/pull/15714