Add CI pipeline to test APX feature

[DeepakRajendrakumaran]

Currently, APX is disabled by default since no targets supporting it is available. CI does not test this feature as of now. It is possible to ensure that we do not run into any asserts etc with this feature by testing with altjit

General instructions used when testing locally are as follows

Build APX branch with following

build.cmd clr+libs -rc Checked -lc Release
src\tests\build.cmd x64 checked generatelayoutonly

Go to jitutils and run bootstrap.cmd

Set following env variables
set DOTNET_MaxVectorTBitwidth=128// This an altjit issue. Without this we run into an assert even in main when compiling vector API on certain machines
set Dotnet_EnableAPX= 1

set Dotnet_JitStressRegs=0x4000 - this is optional. It just forces eGPR usage where possible

Run from runtime repo root.
jit-diff diff --output C:\Dotnet\APX\Output --pmi --diff --corelib --altjit clrjit_unix_x64_x64.dll --diff C:\Dotnet\runtime\artifacts\bin\coreclr\windows.x64.Checked --core_root C:\Dotnet\runtime\artifacts\tests\coreclr\windows.x64.Checked\Tests\Core_Root --arch x64

[dotnet-policy-service]

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[kunalspathak]

related discussion: #108799 (comment)

[kunalspathak]

I didn't know we already have #110672 so we just close this one and add notes in the original issue.