DSA is deprecated, but get this aligned with some of the BN_FLG_CONSTTIME work
going on elsewhere.
Change-Id: I676ceab298a69362bef1b61d6f597c5c90da2ff0
Reviewed-on: https://boringssl-review.googlesource.com/8309
Reviewed-by: Adam Langley <agl@google.com>