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/p1am-100.json

@@ -2,6 +2,5 @@
     "inherits": ["atsamd21g18a"],
     "build-tags": ["sam", "atsamd21g18a", "p1am_100"],
     "flash-command": "bossac -d -i -e -w -v -R --port={port} --offset=0x2000 {bin}",
-    "flash-1200-bps-reset": "true",
-    "default-stack-size": 2048
+    "flash-1200-bps-reset": "true"
 }