Add alignment attribute to packed.

Make packed objects 4 byte aligned or else GCC becomes paranoid and
assumes only byte alignment on whole objects.
Change CHECK in GetCalleeSaveMethod to DCHECK to encourage inlining.

Change-Id: I79f1a13752492413c919b36f4e207e637647834f
diff --git a/src/image.h b/src/image.h
index 852367d..9ed4dc1 100644
--- a/src/image.h
+++ b/src/image.h
@@ -25,7 +25,7 @@
 namespace art {
 
 // header of image files written by ImageWriter, read and validated by Space.
-class PACKED ImageHeader {
+class PACKED(4) ImageHeader {
  public:
   ImageHeader() {}