Cortex-M docs: mark DSP and Scalar targets as supported (pytorch#20224)

### Summary
The target-selectable test infrastructure now builds and exercises the
backend across all three CMSIS-NN instruction-set variants, so the
Target Support table no longer needs the "may work, but untested" caveat
for DSP and pure-C cores. The note now points readers at the
build_test_runner.sh and pytest --cortex-m-target flags used to select a
variant.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils
@Sebastian-Larsson @robell

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: rascani

docs/source/backends/arm-cortex-m/arm-cortex-m-overview.md

@@ -10,13 +10,13 @@ The Arm® Cortex®-M backend accelerates quantized model execution on Arm
 
 The backend targets Arm Cortex-M CPUs via CMSIS-NN, which provides optimized kernel implementations for three instruction set variants:
 
-| Variant      | Description                 | Example CPUs       | Supported |
-|--------------|-----------------------------|--------------------|-----------|
-| MVE (Helium) | M-profile Vector extensions | Cortex-M55, M85    | ✅        |
-| DSP          | DSP extension instructions  | Cortex-M4, M7, M33 | ⬜        |
-| Pure C       | Reference C implementation  | Any Cortex-M       | ⬜        |
+| Variant         | Description                 | Example CPUs           | Supported |
+|-----------------|-----------------------------|------------------------|-----------|
+| MVE (Helium)    | M-profile Vector extensions | Cortex-M55, M85        | ✅        |
+| DSP             | DSP extension instructions  | Cortex-M4, M7, M33     | ✅        |
+| Scalar (Pure C) | Reference C implementation  | Any Cortex-M (M0–M85)  | ✅        |
 
-DSP and pure C variants use the same CMSIS-NN API and may work, but have not been tested.
+The variant is selected from the target CPU's `-mcpu` flag. Build a test runner for a specific target with `backends/cortex_m/test/build_test_runner.sh --target=<cortex-mX>` and run tests against it with `pytest --cortex-m-target=<cortex-mX>`.
 
 ## CMSIS-NN Supported Operators