runtime: print start of GC cycle in gctrace, rather than end

Currently the GODEBUG=gctrace=1 trace line includes "@n.nnns" to
indicate the time that the GC cycle ended relative to the time the
program started. This was meant to be consistent with the utilization
as of the end of the cycle, which is printed next on the trace line,
but it winds up just being confusing and unexpected.

Change the trace line to include the time that the GC cycle started
relative to the time the program started.

Change-Id: I7d64580cd696eb17540716d3e8a74a9d6ae50650
Reviewed-on: https://go-review.googlesource.com/10634Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mgc.go

@@ -1006,7 +1006,7 @@ func gc(mode int) {
 		var sbuf [24]byte
 		printlock()
 		print("gc #", memstats.numgc,
-			" @", string(itoaDiv(sbuf[:], uint64(tEnd-runtimeInitTime)/1e6, 3)), "s ",
+			" @", string(itoaDiv(sbuf[:], uint64(tSweepTerm-runtimeInitTime)/1e6, 3)), "s ",
 			util, "%: ")
 		prev := tSweepTerm
 		for i, ns := range []int64{tScan, tInstallWB, tMark, tMarkTerm, tEnd} {