RFR: 8295282: Use Zicboz/cbo.zero to zero-out memory on RISC-V [v7]

[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 pull request now contains 12 commits:

 - review
 - Merge branch 'master' of github.com:openjdk/jdk into dev/ludovic/upstream-zicboz
 - Explicit use of temp registers
 - fixup! Add -XX:CacheLineSize= to set cache line size
 - fixup! Add -XX:CacheLineSize= to set cache line size
 - fixup! Add -XX:CacheLineSize= to set cache line size
 - fixup! Add -XX:CacheLineSize= to set cache line size
 - Add -XX:CacheLineSize= to set cache line size
 - Fix comment
 - Fix alignement
 - ... and 2 more: https://git.openjdk.org/jdk/compare/ef62b614...ae39b0c0

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Changes: https://git.openjdk.org/jdk/pull/10718/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10718&range=06
  Stats: 165 lines in 9 files changed: 149 ins; 4 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