The padding check functions will need to tweak their calling conventions
and the constant-time helpers, so leaving those alone for now. These
were the easy ones.

BUG=22

Change-Id: Ia00e41e26a134de17d56be3def5820cb042794e1
Reviewed-on: https://boringssl-review.googlesource.com/14265
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>