main: use gdb-multiarch for debugging Cortex-M chips

Previously this was set to arm-none-eabi-gdb, but that has been replaced
by gdb-multiarch in recent Debian/Ubuntu versions.

targets/cortex-m.json

@@ -24,5 +24,5 @@
 		"src/device/arm/cortexm.s",
 		"src/runtime/scheduler_cortexm.S"
 	],
-	"gdb": "arm-none-eabi-gdb"
+	"gdb": "gdb-multiarch"
 }