runtime: trace sweep completion in gcpacertrace mode

Change-Id: I7991612e4d064c15492a39c19f753df1db926203
Reviewed-on: https://go-review.googlesource.com/17747
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mgcsweep.go

@@ -95,6 +95,9 @@ func sweepone() uintptr {
 		if idx >= uint32(len(work.spans)) {
 			mheap_.sweepdone = 1
 			_g_.m.locks--
+			if debug.gcpacertrace > 0 && idx == uint32(len(work.spans)) {
+				print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", mheap_.spanBytesAlloc>>20, "MB of spans; swept ", mheap_.pagesSwept, " pages\n")
+			}
 			return ^uintptr(0)
 		}
 		s := work.spans[idx]