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>

src/runtime/runtime2.go

@@ -421,6 +421,9 @@ type schedt struct {
 	lastpoll   uint64
 
 	profilehz int32 // cpu profiling rate
+
+	procresizetime int64 // nanotime() of last change to gomaxprocs
+	totaltime      int64 // ∫gomaxprocs dt up to procresizetime
 }
 
 // The m->locked word holds two pieces of state counting active calls to LockOSThread/lockOSThread.