There is currently only one assembly implementation of AES
(amd64). While it is possible to fit other implementations to the
same pattern it complicates the code. For example s390x does not
use expanded keys, so having enc and dec in the aesCipher struct
is confusing.

By separating out the asm implementations we can more closely
match the data structures to the underlying implementation. This
also opens the door for AES implementations that support block
cipher modes other than GCM (e.g. CTR and CBC).

This commit changes BenchmarkExpandKey to test the go
implementation of key expansion. It might be better to have some
sort of 'initialisation' benchmark instead to cover the startup
costs of the assembly implementations (which might be doing
key expansion in a different way, or not at all).

Change-Id: I094a7176b5bbe2177df73163a9c0b711a61c12d6
Reviewed-on: https://go-review.googlesource.com/22193
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>