[dev.garbage] runtime: ignore objects in dead spans

We still don't know why this is happening.

LGTM=rlh
R=rlh
CC=golang-codereviews
https://codereview.appspot.com/169990043
[dev.garbage] cmd/gc, runtime: add locks around print statements

Now each C printf, Go print, or Go println is guaranteed
not to be interleaved with other calls of those functions.
This should help when debugging concurrent failures.

LGTM=rlh
R=rlh
CC=golang-codereviews
https://codereview.appspot.com/169120043