runtime: print SP/FP on bad pointer crashes

If the bad pointer is on a stack, this makes it possible to find the
frame containing the bad pointer.

Change-Id: Ieda44e054aa9ebf22d15d184457c7610b056dded
Reviewed-on: https://go-review.googlesource.com/37858
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mbitmap.go

@@ -425,6 +425,7 @@ func heapBitsForObject(p, refBase, refOff uintptr) (base uintptr, hbits heapBits
 				print("runtime: found in object at *(", hex(refBase), "+", hex(refOff), ")\n")
 				gcDumpObject("object", refBase, refOff)
 			}
+			getg().m.traceback = 2
 			throw("found bad pointer in Go heap (incorrect use of unsafe or cgo?)")
 		}
 		return

src/runtime/mgcmark.go

@@ -1230,6 +1230,7 @@ func greyobject(obj, base, off uintptr, hbits heapBits, span *mspan, gcw *gcWork
 			// Dump the object
 			gcDumpObject("obj", obj, ^uintptr(0))
 
+			getg().m.traceback = 2
 			throw("checkmark found unmarked object")
 		}
 		if hbits.isCheckmarked(span.elemsize) {
@@ -1244,6 +1245,7 @@ func greyobject(obj, base, off uintptr, hbits heapBits, span *mspan, gcw *gcWork
 			print("runtime: marking free object ", hex(obj), " found at *(", hex(base), "+", hex(off), ")\n")
 			gcDumpObject("base", base, off)
 			gcDumpObject("obj", obj, ^uintptr(0))
+			getg().m.traceback = 2
 			throw("marking free object")
 		}