arm: print an error when a HardFault occurs

This is very useful for debugging. It differentiates between a stack
overflow and other errors (because it's easy to see when a stack
overflow occurs) and prints the old stack pointer and program counter if
available.

targets/cortex-m.json

@@ -19,5 +19,8 @@
 	"ldflags": [
 		"--gc-sections"
 	],
+	"extra-files": [
+		"src/device/arm/cortexm.s"
+	],
 	"gdb": "arm-none-eabi-gdb"
 }