runtime: improve select benchmarks 1. Add select on sync channels benchmark. 2. Make channels in BenchmarkSelectNonblock shared. With GOMAXPROCS=1 it is the same, but with GOMAXPROCS>1 it becomes a more interesting benchmark.
runtime: improve select benchmarks
1. Add select on sync channels benchmark.
2. Make channels in BenchmarkSelectNonblock shared.
With GOMAXPROCS=1 it is the same, but with GOMAXPROCS>1
it becomes a more interesting benchmark.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews
https://codereview.appspot.com/115780043

wesolows/golang,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old