RFR: 8299844: RISC-V: Implement _onSpinWait intrinsic [v2]

[Yadong Wang]

> Implement _onSpinWait() intrinsic for RISC-V, and it can be implemented by using PAUSE instruction from Zihintpause Extension, which is ratified and a mandatory extension of RVA22U64 (https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc#rva22u64-mandatory-extensions).
> Software can use the PAUSE instruction to reduce energy consumption while executing spinwait
> code sequences (https://github.com/riscv/riscv-isa-manual/blob/master/src/zihintpause.tex).
> 
> Add a test case of test/hotspot/jtreg/compiler/onSpinWait/TestOnSpinWaitRISCV64.java, passed on QEMU.

Yadong Wang has updated the pull request incrementally with one additional commit since the last revision:

  fix copyright year

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11921/files
  - new: https://git.openjdk.org/jdk/pull/11921/files/7d9861e1..4f8a526e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11921&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11921&range=00-01

  Stats: 17 lines in 9 files changed: 5 ins; 4 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/11921.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11921/head:pull/11921

PR: https://git.openjdk.org/jdk/pull/11921