RFR: 8295282: Use Zicboz/cbo.zero to zero-out memory on RISC-V [v15]

[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:

  Make sure DEFAULT_CACHE_LINE_SIZE is defined for risc-v

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10718/files
  - new: https://git.openjdk.org/jdk/pull/10718/files/021f53d9..57910650

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10718&range=14
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10718&range=13-14

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 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