Migrate some stm32 examples to use toolsets (#94)

Updates the stm32-uart-echo and stm32-neopixel examples to use a swiftpm
toolset.json to specify various compiler and linker flags. Also updates
the Makefiles to leverage swiftpm to link the executable instead of
manually linking using `ld` directly.

Author: rauhul

.github/workflows/build-stm.yml

@@ -17,7 +17,7 @@ jobs:
       fail-fast: false
       matrix:
         example: [stm32-blink]
-        swift: [swift-DEVELOPMENT-SNAPSHOT-2024-12-04-a]
+        swift: [swift-DEVELOPMENT-SNAPSHOT-2025-03-04-a]
 
     steps:
       - name: Checkout repo