RFR: 8295282: Use Zicboz/cbo.zero to zero-out memory on RISC-V [v17]

[Ludovic Henry]

> Similarly to AArch64 DC.ZVA, the RISC-V Zicboz [1] extension provides the cbo.zero [2] instruction that allows to zero out memory a cache-line at a time. This should be faster than storing zeroes 64bits at a time.
> 
> [1] https://github.com/riscv/riscv-CMOs
> [2] https://github.com/riscv/riscv-CMOs/blob/master/cmobase/Zicboz.adoc#insns-cbo_zero

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

 - cleanup
 - Revert "Enable Block Zeroing when Usez64b is enabled only"
   
   This reverts commit 021f53d9ec3d87be2d85b3a159db30e5960c47e0.
 - Fix prefetch_w encoding
   
   Reference at:
   https://github.com/riscv/riscv-CMOs/blob/master/cmobase/insns/prefetch.i.adoc
   https://github.com/riscv/riscv-CMOs/blob/master/cmobase/insns/prefetch.r.adoc
   https://github.com/riscv/riscv-CMOs/blob/master/cmobase/insns/prefetch.w.adoc

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10718/files
  - new: https://git.openjdk.org/jdk/pull/10718/files/3156dc9c..0a12b2c8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10718&range=16
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10718&range=15-16

  Stats: 28 lines in 5 files changed: 16 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/10718.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10718/head:pull/10718

PR: https://git.openjdk.org/jdk/pull/10718