runtime: print sweep ratio if gcpacertrace>0

Change-Id: I5217bf4b75e110ca2946e1abecac6310ed84dad5
Reviewed-on: https://go-review.googlesource.com/21205
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgcsweep.go

@@ -96,7 +96,7 @@ func sweepone() uintptr {
 			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")
+				print("pacer: sweep done at heap size ", memstats.heap_live>>20, "MB; allocated ", mheap_.spanBytesAlloc>>20, "MB of spans; swept ", mheap_.pagesSwept, " pages at ", mheap_.sweepPagesPerByte, " pages/byte\n")
 			}
 			return ^uintptr(0)
 		}