Bug 1480558 - part 2 - don't add MOZ_DEBUG_FLAGS to ASFLAGS on aarch64 windows; r=mshal

armasm64 doesn't accept the same options as its x86-ish counterparts,
and passing options it doesn't understand causes assembly to fail.  So
let's just not pass any flags to the assembler for the moment.

python/mozbuild/mozbuild/frontend/context.py

@@ -373,6 +373,10 @@ class AsmFlags(BaseCompileFlags):
                     debug_flags += ['-g', 'cv8']
                 elif self._context.config.substs.get('OS_ARCH') != 'Darwin':
                     debug_flags += ['-g', 'dwarf2']
+            elif (self._context.config.substs.get('OS_ARCH') == 'WINNT' and
+                  self._context.config.substs.get('CPU_ARCH') == 'aarch64'):
+                # armasm64 accepts a paucity of options compared to ml/ml64.
+                pass
             else:
                 debug_flags += self._context.config.substs.get('MOZ_DEBUG_FLAGS', '').split()
         return debug_flags