Change aes_set_gcm_key to be more flexible.

Firstly, it was odd that AES-NI was a special case. Secondly, I have a
need coming up for being able to get the block function and not create a
GCM context.

Change-Id: Ie87de5e7ea42dc042d302c5eafecbc6af03c714b
Reviewed-on: https://boringssl-review.googlesource.com/3910
Reviewed-by: Adam Langley <agl@google.com>