Uses AES hardware instructions on 386/amd64 to implement
a fast hash function.  Incorporates a random key to
thwart hash collision DOS attacks.
Depends on CL#7548043 for new assembly instructions.

Update #3885
Helps some by making hashing faster.  Go time drops from
0.65s to 0.51s.

R=rsc, r, bradfitz, remyoudompheng, khr, dsymonds, minux.ma, elias.naur
CC=golang-dev
https://golang.org/cl/7543043