RFR: 8339527: Adjust threshold for MemorySegment::fill native invocation

[Per Minborg]

This PR proposes to increase the threshold for `MemorySegment::fill` operations 
on AArch-64 platforms.

Performance looks good:

![image](https://github.com/user-attachments/assets/56a83541-cb84-4a37-a914-fdddfde343c6)

![image](https://github.com/user-attachments/assets/ff2c33e0-b300-482d-b93e-217a11e07202)

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Commit messages:
 - fill-threshold

Changes: https://git.openjdk.org/jdk/pull/24510/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24510&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339527
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/24510.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24510/head:pull/24510

PR: https://git.openjdk.org/jdk/pull/24510