Make primitive getter/setters consistently signed.

Use QuasiAtomic for Cas64, which fixes the MIPS build of ART.

Change-Id: Id1dba264b148a7ce9bcc637159ec733cfbd88b70
diff --git a/runtime/mirror/string.h b/runtime/mirror/string.h
index a82b26c..406c5a3 100644
--- a/runtime/mirror/string.h
+++ b/runtime/mirror/string.h
@@ -112,7 +112,7 @@
 
  private:
   void SetHashCode(int32_t new_hash_code) {
-    DCHECK_EQ(0u, GetField32(OFFSET_OF_OBJECT_MEMBER(String, hash_code_), false));
+    DCHECK_EQ(0, GetField32(OFFSET_OF_OBJECT_MEMBER(String, hash_code_), false));
     SetField32(OFFSET_OF_OBJECT_MEMBER(String, hash_code_), new_hash_code, false);
   }