IMPALA-1111: Fix previous DecimalVal alignment patch

The compiler was changing the memcpy into an operation requiring
16-byte alignment.

Change-Id: Ic76ecffe231f82c4dff0edcabe5b9d26e5636bbe
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4508
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins