Tools: Fix Microbit releases

### Description

I noticed that there was quite a bit missing from the mbed2 release of
the microbit today. That's because the microbit uses ARM by default and
the GCC_ARM small library. The release script was trying to release for
ARM using scan rules for uARM. Turns out we're stuck with 2 configuration
parameters for the same thing: `default_lib` for GCC and `default_toolchain`
for ARM. Dang

### Pull request type

    [x] Fix
    [ ] Refactor
    [ ] Target update
    [ ] Functionality change
    [ ] Breaking change

tools/toolchains/arm.py

@@ -125,7 +125,7 @@ class ARM(mbedToolchain):
             })
 
     def _get_toolchain_labels(self):
-        if getattr(self.target, "default_lib", "std") == "small":
+        if getattr(self.target, "default_toolchain", "ARM") == "uARM":
             return ["ARM", "ARM_MICRO"]
         else:
             return ["ARM", "ARM_STD"]