[jdk17u-dev] RFR: 8357968: RISC-V: Interpreter volatile reference stores with G1 are not sequentially consistent

[Feilong Jiang]

Fixes RISC-V memory ordering problem.
This patch is not clean because [JDK-8293290](https://bugs.openjdk.org/browse/JDK-8293290) adds a third temporary register to `store_heap_oop`, which is introduced in JDK20. 
In JDK17, `do_oop_store` also clobbers the same `x13` as in mailine. The current fix is similar to aarch64 fixes in mainline, but puts the flags in `x15` instead, to avoid the clobber of the volatile bits.

Additional testing:
- [x] linux-riscv64 server release, jcstress RefDekker test now passes
- [ ] linux-riscv64 server release, Tier1-3

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Commit messages:
 - Backport c5a1543ee3e68775f09ca29fb07efd9aebfdb33e

Changes: https://git.openjdk.org/jdk17u-dev/pull/3678/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=3678&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357968
  Stats: 8 lines in 1 file changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/3678.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/3678/head:pull/3678

PR: https://git.openjdk.org/jdk17u-dev/pull/3678