crypto/rc4: faster amd64 implementation

XOR key into data 128 bits at a time instead of 64 bits
and pipeline half of state loads. Rotate loop to allow
single-register indexing for state[i].

On a MacBookPro10,2 (Core i5):

benchmark           old ns/op    new ns/op    delta
BenchmarkRC4_128          412          224  -45.63%
BenchmarkRC4_1K          3179         1613  -49.26%
BenchmarkRC4_8K         25223        12545  -50.26%

benchmark            old MB/s     new MB/s  speedup
BenchmarkRC4_128       310.51       570.42    1.84x
BenchmarkRC4_1K        322.09       634.48    1.97x
BenchmarkRC4_8K        320.97       645.32    2.01x

For comparison, on the same machine, openssl 0.9.8r reports
its rc4 speed as somewhat under 350 MB/s for both 1K and 8K
(it is operating 64 bits at a time).

On an Intel Xeon E5520:

benchmark           old ns/op    new ns/op    delta
BenchmarkRC4_128          418          259  -38.04%
BenchmarkRC4_1K          3200         1884  -41.12%
BenchmarkRC4_8K         25173        14529  -42.28%

benchmark            old MB/s     new MB/s  speedup
BenchmarkRC4_128       306.04       492.48    1.61x
BenchmarkRC4_1K        319.93       543.26    1.70x
BenchmarkRC4_8K        321.61       557.20    1.73x

For comparison, on the same machine, openssl 1.0.1
reports its rc4 speed as 587 MB/s for 1K and 601 MB/s for 8K.

R=agl
CC=golang-dev
https://codereview.appspot.com/7865046