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

[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

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Commit messages:
 - 8295282: Use Zicboz/cbo.zero to zero-out memory on RISC-V

Changes: https://git.openjdk.org/jdk/pull/10718/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10718&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295282
  Stats: 114 lines in 8 files changed: 112 ins; 1 del; 1 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