targets: bump cortex-m-qemu default stack size

targets/cortex-m-qemu.json

@@ -2,6 +2,7 @@
 	"inherits": ["cortex-m3"],
 	"build-tags": ["qemu", "lm3s6965"],
 	"linkerscript": "targets/lm3s6965.ld",
+        "default-stack-size": 4096,
 	"extra-files": [
 		"targets/cortex-m-qemu.s"
 	],