runtime: debug code to panic when marking a free object

This adds debug code enabled in gccheckmark mode that panics if we
attempt to mark an unallocated object. This is a common issue with the
hybrid barrier when we're manipulating uninitialized memory that
contains stale pointers. This also tends to catch bugs that will lead
to "sweep increased allocation count" crashes closer to the source of
the bug.

Change-Id: I443ead3eac6f316a46f50b106078b524cac317f4
Reviewed-on: https://go-review.googlesource.com/31761Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgcmark.go

@@ -1294,6 +1294,13 @@ func greyobject(obj, base, off uintptr, hbits heapBits, span *mspan, gcw *gcWork
 			throw("setCheckmarked and isCheckmarked disagree")
 		}
 	} else {
+		if debug.gccheckmark > 0 && span.isFree(objIndex) {
+			print("runtime: marking free object ", hex(obj), " found at *(", hex(base), "+", hex(off), ")\n")
+			gcDumpObject("base", base, off)
+			gcDumpObject("obj", obj, ^uintptr(0))
+			throw("marking free object")
+		}
+
 		// If marked we have nothing to do.
 		if mbits.isMarked() {
 			return