crypto/cipher: Support unusual GCM nonce lengths

crypto/cipher: Support unusual GCM nonce lengths

GCM is traditionally used with a 96-bit nonce, but the standard allows
for nonces of any size. Non-standard nonce sizes are required in some
protocols, so add support for them in crypto/cipher's GCM
implementation.

Change-Id: I7feca7e903eeba557dcce370412b6ffabf1207ab
Reviewed-on: https://go-review.googlesource.com/8946
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>