RFR: 8315841: RISC-V: Check for hardware TSO support [v5]

[Ludovic Henry]

> With the Ztso extension [1], some hardware will support TSO on RISC-V. That allows us to reduce the generation of memory fences, given the stronger memory model compared to RVWMO.
> 
> [1] https://github.com/riscv/riscv-isa-manual/blob/6dcbc6da9ada01f0f57da83cda6059bdec57619f/src/ztso-st-ext.adoc#L1

Ludovic Henry has updated the pull request incrementally with two additional commits since the last revision:

 - fixup! 8315841: RISC-V: Check for hardware TSO support
 - fixup! 8315841: RISC-V: Check for hardware TSO support

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15613/files
  - new: https://git.openjdk.org/jdk/pull/15613/files/5f80c7ca..c41c3e74

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15613&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15613&range=03-04

  Stats: 6 lines in 1 file changed: 4 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15613.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15613/head:pull/15613

PR: https://git.openjdk.org/jdk/pull/15613