runtime: prefetch next block in mallocgc
runtime: prefetch next block in mallocgc

json-1
cputime                  99600000     98600000      -1.00%
time                    100005493     98859693      -1.15%

garbage-1
cputime                  15760000     15440000      -2.03%
time                     15791759     15471701      -2.03%

LGTM=khr
R=golang-codereviews, gobot, khr, dave
CC=bradfitz, golang-codereviews, iant
https://codereview.appspot.com/57310043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang