RFR: 8295282: Use Zicboz/cbo.zero to zero-out memory on RISC-V [v2]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Fix comment
 - Fix alignement
 - Merge branch 'master' of github.com:openjdk/jdk into dev/ludovic/upstream-zicboz
 - 8295282: Use Zicboz/cbo.zero to zero-out memory on RISC-V
   
   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

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10718/files
  - new: https://git.openjdk.org/jdk/pull/10718/files/5daadc29..845d0e3b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10718&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10718&range=00-01

  Stats: 3715 lines in 108 files changed: 2137 ins; 902 del; 676 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