runtime: preempt goroutines for GC
The last patch for preemptive scheduler,
with this change stoptheworld issues preemption
requests every 100us.
Update issue 543.

R=golang-dev, daniel.morsing, rsc
CC=golang-dev
https://codereview.appspot.com/10264044
runtime: introduce GODEBUG env var
Currently it replaces GOGCTRACE env var (GODEBUG=gctrace=1).
The plan is to extend it with other type of debug tracing,
e.g. GODEBUG=gctrace=1,schedtrace=100.

R=rsc
CC=bradfitz, daniel.morsing, gobot, golang-dev
https://codereview.appspot.com/10026045