net, runtime: enable runtime-integrated pollster on Linux/ARM.

Results from linux/arm on a Samsung Chromebook (from dfc):
localhost(~/go/src/pkg/net) % ~/go/misc/benchcmp {old,new}.txt
benchmark                           old ns/op    new ns/op    delta
BenchmarkTCP4OneShot                   568840       350526  -38.38%
BenchmarkTCP4OneShot-2                 359054       206708  -42.43%
BenchmarkTCP4OneShotTimeout            637464       363550  -42.97%
BenchmarkTCP4OneShotTimeout-2          374255       216695  -42.10%
BenchmarkTCP4Persistent                184974        64984  -64.87%
BenchmarkTCP4Persistent-2              109902        47195  -57.06%
BenchmarkTCP4PersistentTimeout         210039        64789  -69.15%
BenchmarkTCP4PersistentTimeout-2       124284        43374  -65.10%
BenchmarkTCP6OneShot                   672278       362116  -46.14%
BenchmarkTCP6OneShot-2                 383631       216400  -43.59%
BenchmarkTCP6OneShotTimeout            680740       378306  -44.43%
BenchmarkTCP6OneShotTimeout-2          397524       230152  -42.10%
BenchmarkTCP6Persistent                172346        65292  -62.12%
BenchmarkTCP6Persistent-2              106229        42096  -60.37%
BenchmarkTCP6PersistentTimeout         161149        65138  -59.58%
BenchmarkTCP6PersistentTimeout-2       152276        44548  -70.75%

R=golang-dev, dave, bradfitz, dvyukov, rsc
CC=golang-dev
https://codereview.appspot.com/7820045
runtime: scan the type of an interface value

R=golang-dev, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/7744047

Committer: Russ Cox <rsc@golang.org>