Fixes DBC build for Mac

runtime/vm/os_macos.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
 #if defined(TARGET_OS_MACOS)
 
 #include "vm/os.h"
 
 #include <errno.h>  // NOLINT
@@ skipping 151 matching lines @@
 #if TARGET_ARCH_ARM
   // Even if we generate code that maintains a stronger alignment, we cannot
   // assert the stronger stack alignment because C++ code will not maintain it.
   return 8;
 #elif TARGET_ARCH_ARM64
   return 16;
 #elif TARGET_ARCH_IA32
   return 16;  // iOS simulator
 #elif TARGET_ARCH_X64
   return 16;  // iOS simulator
+#elif TARGET_ARCH_DBC
+  return 16;
 #else
 #error Unimplemented
 #endif
 #else  // TARGET_OS_IOS
   // OS X activation frames must be 16 byte-aligned; see "Mac OS X ABI
   // Function Call Guide".
   return 16;
 #endif  // TARGET_OS_IOS
 }
 
@@ skipping 205 matching lines @@
 }
 
 
 void OS::Exit(int code) {
   exit(code);
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)