The memcpy of a pointer looks like a typo, though it isn't. Instead,
transcribe what the functions expect into a union and let C fill it in.

Change-Id: Iba4c824295e8908c5bda68ac35673040a8cff116
Reviewed-on: https://boringssl-review.googlesource.com/18744
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>