crypto/aes: move ppc64le to cipher_asm.go

crypto/aes: move ppc64le to cipher_asm.go

Move the aesCipherGCM struct definition into cipher_asm.go, it is
needed to compile this file, but isn't used on PPC64.

Also, generate a KeySizeError if the key length is not supported
as was done in the ppc64le implementation, and is done in the
generic code.

Change-Id: I025fc63d614b57dac65a18d1ac3dbeec99356292
Reviewed-on: https://go-review.googlesource.com/c/go/+/399254
Reviewed-by: Filippo Valsorda <valsorda@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Paul Murphy <murp@ibm.com>