RFR: 8321282: RISC-V: SpinPause() not implemented

[Robbin Ehn]

Hi, please consider.

Tested tier1 and targeted via gtest WaitBarrier which uses SpinPause(). (with UseZihintpause default true)

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Commit messages:
 - Added SpinPause

Changes: https://git.openjdk.org/jdk/pull/17820/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17820&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321282
  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17820.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17820/head:pull/17820

PR: https://git.openjdk.org/jdk/pull/17820