runtime: fix deadlock when gctrace
Calling ReadMemStats which does stoptheworld on m0 holding locks
was not a good idea.
Stoptheworld holding locks is a recipe for deadlocks (added check for this).
Stoptheworld on g0 may or may not work (added check for this as well).
As far as I understand scavenger will print incorrect numbers now,
as stack usage is not subtracted from heap. But it's better than deadlocking.

LGTM=khr
R=golang-codereviews, rsc, khr
CC=golang-codereviews, rlh
https://codereview.appspot.com/124670043
runtime: init GC later
Init GC later as it needs to read GOGC env var.
Fixes issue 8562.

LGTM=daniel.morsing, rsc
R=golang-codereviews, daniel.morsing, rsc
CC=golang-codereviews, khr, rlh
https://codereview.appspot.com/130990043