runtime: add cumulative GC CPU % to gctrace line

runtime: add cumulative GC CPU % to gctrace line

This tracks both total CPU time used by GC and the total time
available to all Ps since the beginning of the program and uses this
to derive a cumulative CPU usage percent for the gctrace line.

Change-Id: Ica85372b8dd45f7621909b325d5ac713a9b0d015
Reviewed-on: https://go-review.googlesource.com/8350
Reviewed-by: Russ Cox <rsc@golang.org>