runtime: fix race builders
Do not run Syscall benchmarks under race detector,
they split stack in syscall status.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/12093045
runtime: enable goroutine preemption
All known issues with preemption have beed fixed.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/12008044