and GCM on 386 and amd64

Intel(R) Core(TM) i5-2540M CPU @ 2.60GHz:

benchmark                    old MB/s     new MB/s  speedup
BenchmarkAESGCMSeal1K           82.39        92.05    1.12x
BenchmarkAESGCMOpen1K           82.28        91.88    1.12x
BenchmarkAESCFBEncrypt1K       141.54       277.59    1.96x
BenchmarkAESCFBDecrypt1K       133.06       278.07    2.09x
BenchmarkAESOFB1K              160.51       380.24    2.37x
BenchmarkAESCTR1K              164.07       429.25    2.62x
BenchmarkAESCBCEncrypt1K       170.99       263.74    1.54x
BenchmarkAESCBCDecrypt1K       124.96       249.14    1.99x

Fixes #6741.

R=agl, dave, agl
CC=golang-dev
https://golang.org/cl/24250044