Add missing mac and ciphers algos

This MR cherry-picks the following PRs:

• https://github.com/golang/crypto/pull/18
• https://github.com/golang/crypto/pull/129
• https://github.com/golang/crypto/pull/127

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• Adds support for hmac-sha2-512-etm@openssh.com and hmac-sha2-512 macs
• Adds support for aes256-gcm@openssh.com cipher