RFR: 8300253: Introduce AArch64 nzcv accessors

[Erik Österlund]

Some code, such as verification code, might want to run without changing the state of the system. To do that, it's useful to be able to get and set the nzcv flags. This enhancement aims at adding accessors for that.

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Commit messages:
 - 8300253: Introduce AArch64 nzcv accessors

Changes: https://git.openjdk.org/jdk/pull/12038/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12038&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300253
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12038.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12038/head:pull/12038

PR: https://git.openjdk.org/jdk/pull/12038