cortexm: bump default stack size to 2048 bytes

Previously it was 1024 bytes, which occasionally ran into a stack
overflow. I hope that 2048 bytes will be enough for most purposes.

I've also removed some 2048-byte stack size settings in JSON files,
which are unnecessary now that the parent (cortex-m.json) sets them.

targets/feather-stm32f405.json

@@ -2,7 +2,6 @@
   "inherits": ["cortex-m4"],
   "build-tags": ["feather_stm32f405", "stm32f405", "stm32f4", "stm32"],
   "automatic-stack-size": false,
-  "default-stack-size": 1024,
   "linkerscript": "targets/stm32f405.ld",
   "extra-files": [
     "src/device/stm32/stm32f405.s"