runtime: improve "P has cached GC work" debug info

For #27993.

Change-Id: I20127e8a9844c2c488f38e1ab1f8f5a27a5df03e
Reviewed-on: https://go-review.googlesource.com/c/149968
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mgc.go

@@ -1934,6 +1934,19 @@ func gcMark(start_time int64) {
 
 		gcw := &p.gcw
 		if !gcw.empty() {
+			printlock()
+			print("runtime: P ", p.id, " flushedWork ", gcw.flushedWork)
+			if gcw.wbuf1 == nil {
+				print(" wbuf1=<nil>")
+			} else {
+				print(" wbuf1.n=", gcw.wbuf1.nobj)
+			}
+			if gcw.wbuf2 == nil {
+				print(" wbuf2=<nil>")
+			} else {
+				print(" wbuf2.n=", gcw.wbuf2.nobj)
+			}
+			print("\n")
 			throw("P has cached GC work at end of mark termination")
 		}
 		// There may still be cached empty buffers, which we