The exponent is secret, so we should be using the consttime variant. See
also upstream's f9cbf470180841966338db1f4c28d99ec4debec4.
Change-Id: I233d4223ded5b80711d7c8f906e3579c36b24cd0
Reviewed-on: https://boringssl-review.googlesource.com/20924
Reviewed-by: Adam Langley <agl@google.com>