Add CI support for testing APX

[BruceForstall]

Add testing support in the CI system for testing APX.

There are three basic phases:

1. Before we have any APX hardware in the CI, what testing can we do? Assertion checking of the new EGPR/REX2/EVEX/If-Convert code paths using an altjit with those features enabled and a SPMI replay would be useful for assertion checking.
2. When there are a few APX machines in CI, we can add limited, targeted executable testing.
3. Eventually, if a large number or all CI test machines are APX enabled, we can run the full set of testing (including stress) with APX, but then we will also need to disable APX to ensure that scenario still works. Perf testing will also need to include APX.

There exists a runtime-coreclr jitstress-isas-avx512 pipeline (https://dev.azure.com/dnceng-public/public/_build?definitionId=235). Given all our CI machines are AVX-512 capable, do we still need this pipeline? Can this pipeline form a template to create a runtime-coreclr jitstress-isas-apx pipeline?

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[MichalPetryka]

1. Before we have any APX hardware in the CI, what testing can we do?

Could there be a test pipeline running with Intel® Software Development Emulator (Intel® SDE) that has all ISAs emulated to be supported and runs the tests there?

[BruceForstall]

Could there be a test pipeline running with Intel® Software Development Emulator (Intel® SDE) that has all ISAs emulated to be supported and runs the tests there?

It might be possible, but it is preferable to avoid this due to cost (development cost, runtime cost) and the almost inevitable oddities of the emulator. We would prefer to just wait until hardware is available.

[kunalspathak]

Just to make sure we get coverage on JitStressRegs mentioned in #112239 (comment)