microbit: changed default target to gcc

gcc is more widely used in the community due to being open source and free...
changed the default compiler to suit the common case.

.yotta.json

@@ -1,5 +1,5 @@
 {
   "build": {
-    "target": "bbc-microbit-classic-armcc,*"
+    "target": "bbc-microbit-classic-gcc,*"
   }
-}
+}