RFR: 8338967: Improve performance for MemorySegment::fill [v6]

[Per Minborg]

> The performance of the `MemorySegment::fil` can be improved by replacing the `checkAccess()` method call with calling `checkReadOnly()` instead (as the bounds of the segment itself do not need to be checked).
> 
> Also, smaller segments can be handled directly by Java code rather than transitioning to native code.
> 
> Here is how the `MemorySegment::fill` performance is improved by this PR:
> 
> ![image](https://github.com/user-attachments/assets/92a0bcf2-f5b0-4a91-9c02-39423f870209)
> 
> Operations involving 8 or more bytes are delegated to native code whereas smaller segments are handled via a switch rake.
> 
> It should be noted that `Arena::allocate` is using `MemorySegment::fil`. Hence, this PR will also have a positive effect on memory allocation performance.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Switch to bit checking instead of switch statement

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20712/files
  - new: https://git.openjdk.org/jdk/pull/20712/files/6fb6eefa..e2a52912

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20712&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20712&range=04-05

  Stats: 33 lines in 1 file changed: 6 ins; 15 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/20712.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20712/head:pull/20712

PR: https://git.openjdk.org/jdk/pull/20712