Fix configuration of embedded runtime for Morello

This fixes an erroneous early return in MorelloTarget.system_ads
which caused the system.ads for the 'embedded' runtime to never be
added to the list.

Issue: eng/toolchain/bb-runtimes#18

aarch64/__init__.py

@@ -101,7 +101,7 @@ def compiler_switches(self):
 
     @property
     def system_ads(self):
-        return {
+        result = {
             'light': 'system-xi-arm-nxstack-light.ads',
             'light-tasking': 'system-xi-arm-nxstack-light-tasking.ads'
         }