Cortex-M backend: fix Zephyr hello-executorch CI command

The Zephyr `hello-executorch` README has `<!-- RUN -->` directives that
the test-arm-backend-zephyr CI extracts and executes verbatim. The
prior wording used `--target=cortex-m55+int8`, which the AOT compiler
no longer accepts after the +int8 drop. Update both code blocks (and
the surrounding prose) to use the new bare `--target=cortex-m55`
spelling.

Authored with Claude.

Author: rascani

zephyr/samples/hello-executorch/README.md

@@ -51,10 +51,10 @@ west build -b mps3/corstone300/fvp modules/lib/executorch/zephyr/samples/hello-e
 Prepare the Cortex-M55 PTE model
 <!-- RUN test_cortex-m55_generate_pte -->
 ```
-python -m modules.lib.executorch.backends.arm.scripts.aot_arm_compiler --model_name=modules/lib/executorch/zephyr/samples/hello-executorch/models/add.py --quantize --target=cortex-m55+int8 --output=add_m55.pte
+python -m modules.lib.executorch.backends.arm.scripts.aot_arm_compiler --model_name=modules/lib/executorch/zephyr/samples/hello-executorch/models/add.py --quantize --target=cortex-m55 --output=add_m55.pte
 ```
 
-`--target=cortex-m55+int8` selects the Cortex-M/CMSIS-NN portable kernel path (no NPU delegation). This produces a `.pte` optimized for Cortex-M55 with INT8 quantization.
+`--target=cortex-m55` plus `--quantize` selects the Cortex-M/CMSIS-NN portable kernel path (no NPU delegation). This produces a `.pte` optimized for Cortex-M55 with INT8 quantization.
 
 
 #### Build and run
@@ -129,10 +129,10 @@ export PATH=$PATH:~/STMicroelectronics/STM32Cube/STM32CubeProgrammer/bin
 
 Prepare the Cortex-M55 PTE model
 ```
-python -m modules.lib.executorch.backends.arm.scripts.aot_arm_compiler --model_name=modules/lib/executorch/zephyr/samples/hello-executorch/models/add.py --quantize --target=cortex-m55+int8 --output=add_m55.pte
+python -m modules.lib.executorch.backends.arm.scripts.aot_arm_compiler --model_name=modules/lib/executorch/zephyr/samples/hello-executorch/models/add.py --quantize --target=cortex-m55 --output=add_m55.pte
 ```
 
-`--target=cortex-m55+int8` selects the Cortex-M/CMSIS-NN portable kernel path (no NPU delegation). This produces a `.pte` optimized for Cortex-M55 with INT8 quantization.
+`--target=cortex-m55` plus `--quantize` selects the Cortex-M/CMSIS-NN portable kernel path (no NPU delegation). This produces a `.pte` optimized for Cortex-M55 with INT8 quantization.
 
 #### Build and run