RFR: 8295282: Use Zicboz/cbo.zero to zero-out memory on RISC-V [v14]

[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 one additional commit since the last revision:

  Enable Block Zeroing when Usez64b is enabled only

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10718/files
  - new: https://git.openjdk.org/jdk/pull/10718/files/d01f4b1c..021f53d9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10718&range=13
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10718&range=12-13

  Stats: 27 lines in 5 files changed: 0 ins; 18 del; 9 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