runtime: make traceback indicate whether _defer was just allocated

Many of the crashes observed in #27993 involve committing the new
_defer object at the end of newdefer. It would be helpful to know if
the _defer was just allocated or was retrieved from the defer pool. In
order to indicate this in the traceback, this CL duplicates the tail
of newdefer so that the PC/line number will tell us whether d is new
or not.

For #27993.

Change-Id: Icd3e23dbcf00461877bb082b6f18df701149a607
Reviewed-on: https://go-review.googlesource.com/c/154598
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/panic.go

@@ -241,6 +241,15 @@ func newdefer(siz int32) *_defer {
 			total := roundupsize(totaldefersize(uintptr(siz)))
 			d = (*_defer)(mallocgc(total, deferType, true))
 		})
+		if debugCachedWork {
+			// Duplicate the tail below so if there's a
+			// crash in checkPut we can tell if d was just
+			// allocated or came from the pool.
+			d.siz = siz
+			d.link = gp._defer
+			gp._defer = d
+			return d
+		}
 	}
 	d.siz = siz
 	d.link = gp._defer