Sign a tx with "low s" value using OpenSSL
Alternatively, you could use libsecp256k1. This is the code used by Bitcoin Core for signing, and will automatically create low-S signatures (disclaimer: I'm the main author of that library).
Perhaps a Go wrapper exists.

If you stick to OpenSSL, it is possible to manually adjust the S value after signing. This is what Bitcoin Core used to do before v0.10. Here is what it used to do: https://github.com/bitcoin/bitcoin/blob/v0.9.0/src/key.cpp#L204L224