Mark most *Offset helper functions as constexpr.

Making the values compile-time constants will help to
clean up the cpp-define-generator.

Test: test.py -b -g
Change-Id: I612a19a54062784b501bfe4f41c6642d48e0dd21
diff --git a/runtime/runtime.h b/runtime/runtime.h
index ea2eb39..478ff50 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -399,7 +399,7 @@
   QuickMethodFrameInfo GetRuntimeMethodFrameInfo(ArtMethod* method)
       REQUIRES_SHARED(Locks::mutator_lock_);
 
-  static size_t GetCalleeSaveMethodOffset(CalleeSaveType type) {
+  static constexpr size_t GetCalleeSaveMethodOffset(CalleeSaveType type) {
     return OFFSETOF_MEMBER(Runtime, callee_save_methods_[static_cast<size_t>(type)]);
   }