Our use-case for this does not require optimisation at the current time,
so a clean C implementation is fine.
Change-Id: I8f29572c33e8dbcc37961c099c71c14aafc8d0a3
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/45164
Reviewed-by: David Benjamin <davidben@google.com>