runtime: use 3x fewer nanotime calls in garbage collection
runtime: use 3x fewer nanotime calls in garbage collection

Cuts the number of calls from 6 to 2 in the non-debug case.

LGTM=iant
R=golang-codereviews, iant
CC=0intro, aram, golang-codereviews, khr
https://codereview.appspot.com/86040043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang