Fix PACKED for jvalue

Make it only as packed as alignof(mirror::Object*).

Bug: 31532493

Test: mm test-art-host

Change-Id: I242c549bcc9203d3173f204264b0dc2f2f8e3441
diff --git a/runtime/jvalue.h b/runtime/jvalue.h
index 52a0f23..398bfbc 100644
--- a/runtime/jvalue.h
+++ b/runtime/jvalue.h
@@ -29,7 +29,7 @@
 class Object;
 }  // namespace mirror
 
-union PACKED(4) JValue {
+union PACKED(alignof(mirror::Object*)) JValue {
   // We default initialize JValue instances to all-zeros.
   JValue() : j(0) {}