Enable SymbolicSgdLogisticRegression (SymSgdNative) on arm64

[vladimir-aubrecht]

Summary

Enables the SymbolicSgdLogisticRegression trainer (backed by SymSgdNative) on arm/arm64, where Intel MKL is unavailable. Fixes #5798.

SymSGD uses only four CBLAS routines (sdot, saxpy, sdoti, saxpyi). This PR provides them via a tiny, self-contained libMklImports shim built from portable C — with no external BLAS dependency — so it configures and links in the CI cross-compilation sysroots (which ship no OpenBLAS/BLAS).

Changes

• src/Native/MklImportsArm/ (new): implements the four CBLAS routines as plain C loops (-O3 autovectorizes the dense paths to NEON) plus MKL DFTI stubs. No find_package(BLAS) — zero external deps. Symbols are exported explicitly because the native build uses -fvisibility=hidden.
• src/Native/CMakeLists.txt: build MklImportsArm + SymSgdNative on arm.
• src/Native/SymSgdNative/CMakeLists.txt: link SymSgdNative against the shim on arm.
• src/Native/SymSgdNative/SparseBLAS.h: make the CBLAS calling convention portable (__cdecl only on _WIN32).
• Directory.Build.targets: ship libMklImports and libSymSgdNative next to the managed assemblies on arm.
• SymSgdClassificationTrainer.cs: marshal LearnAll's native bool parameters as UnmanagedType.I1. The default 4-byte bool marshalling corrupts later stack arguments and segfaults on arm64 (works on x64 by luck).

Testing

Built native (Release + Debug) and ran the SymSGD trainer tests on Apple Silicon (arm64):

• TestEstimatorSymSgdClassificationTrainer
• TestEstimatorSymSgdInitPredictor
• SimpleTrainAndPredictSymSGD

3 passed, 0 failed. Without the bool marshalling fix, these crash the test host with a SIGSEGV inside LearnAll.

Notes

• The initial arm enablement scaffolding (MklImportsArm skeleton, CMake gating, SparseBLAS.h calling-convention guard) is based on prior work by @anicka-net (credited as co-author).
• Broader MKL consumers in Microsoft.ML.Mkl.Components (OLS, VectorWhitening) and Microsoft.ML.TimeSeries still require a real BLAS/LAPACK and are out of scope here.

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