runtime: remove now arg from timer callback
Cleanup before converting to Go.
Fortunately nobody using it, because it is incorrect:
monotonic runtime time instead of claimed real time.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rsc
https://codereview.appspot.com/129480043
runtime: convert lfstack to Go
It is called from Go only in tests.

LGTM=khr
R=golang-codereviews, khr
CC=golang-codereviews, rlh, rsc
https://codereview.appspot.com/125610043