RFR: 8378107: Data cache zeroing is used even when it is prohibited

[Andrew Haley]

The DC ZVA instruction should not be used when DZP, bit [4] in DCZID_EL0, is set.

It turns out that detection of DZP, bit [4] in DCZID_EL0, never worked. And this was never a problem until now, because everyone supports DC ZVA instructions.

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Commit messages:
 - Merge from master
 - 8378107: Data cache zeroing is used even when it is prohibited

Changes: https://git.openjdk.org/jdk/pull/29762/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29762&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8378107
  Stats: 4 lines in 2 files changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/29762.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29762/head:pull/29762

PR: https://git.openjdk.org/jdk/pull/29762