Have jitstress_isas_x86_evex set PreferredVectorBitWidth=512

[tannergooding]

This resolves #88233 by ensuring the evex stress job runs with DOTNET_PreferredVectorBitWidth=512.

Given we're already stress testing EVEX, ensuring that Vector512.IsHardwareAccelerated reports true in that case matches the general theme of the job.

This also fixes an issue with IsHardwareAccelerated not respecting the right flags/properties and updates the tests to check for the right values, accordingly.

[ghost]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

Issue Details

---

This resolves #88233 by ensuring the evex stress job runs with DOTNET_PreferredVectorBitWidth=512.

Given we're already stress testing EVEX, ensuring that Vector512.IsHardwareAccelerated reports true in that case matches the general theme of the job.

Author:	tannergooding
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[tannergooding]

CC. @dotnet/jit-contrib, as per the top post this resolves the issue where we weren't running CI jobs where Vector512.IsHardwareAccelerated was true and ensures that IsHardwareAccelerated returns true when it was expected to do so.

[BruceForstall]

Do you want to trigger runtime-coreclr jitstress-isas-avx512?

[tannergooding]

/azp run runtime-coreclr jitstress-isas-avx512

[azure-pipelines]

Azure Pipelines could not run because the pipeline triggers exclude this branch/path.