runtime: adopt race detector for runtime written in Go
Ignore memory access on g0/gsignal.
See the issue for context and explanation.
Fixes issue 8627.

LGTM=khr
R=golang-codereviews, mdempsky, khr
CC=golang-codereviews, rsc
https://codereview.appspot.com/137070043
runtime: remove guard against uninitialized forcegc.g

The race was in the old C code.
The new Go code does not have the race
and does not need the check.

LGTM=bradfitz, dvyukov
R=golang-codereviews, bradfitz, dvyukov
CC=golang-codereviews, rlh
https://codereview.appspot.com/140180043
runtime: relax flaky GC pause test

We often saw GC pauses of 0 ns, not just on Windows.
Google Compute Engine timer granularity might suck
too.

LGTM=rsc
R=rsc, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/140910043