So, for the past 6 months (from Feb. '16), I have been successfully using Xcode as my development environment for ARM cortex-mX devices (mainly cortex-m0). 
 
August '15 I finished creating an Xcode platform for ARM devices copied from the existing iPhone platform (with additional inspiration from Appportable's Android development platform for Xcode) ... unfortunately, it doesn't do native (meaning built-in) debugging. This is primarily for 2 reasons - 1) I haven't figured it out ... yet. But I WILL. And 2) The JLink tools require the use of GDB instead of LLDB, causing all sorts of headaches getting GDB running in the latest edition(s) of Xcode. Again, I WILL solve this. For the time being, I'm using the Segger JLink Debugger app; it's OS X native, and is scriptable, so it can be called from Xcode. 
 
If you would like more information, or have suggestions on how to fix the GDB / JLink problem(s), drop me a line.