This change replace the cmovq scheme with slightly faster SSE2 code.
The SSE2 code was first introduced in Go's curve25519 implementation.
See: https://go-review.googlesource.com/c/39693/

The implementation is basicly copied from the Go assembly.

Change-Id: I25931a421ba141ce33809875699f048b0941c061
Reviewed-on: https://boringssl-review.googlesource.com/16564
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>