Helps reduce contention on sched.goidgen.

benchmark                               old ns/op    new ns/op    delta
BenchmarkCreateGoroutines-16                  259          237   -8.49%
BenchmarkCreateGoroutinesParallel-16          127           43  -66.06%

R=golang-codereviews, dave, bradfitz, khr
CC=golang-codereviews, rsc
https://golang.org/cl/46970043