runtime: print debug info on "base out of range"

This adds debugging information when we panic with "heapBitsForSpan:
base out of range".

Updates #20259.

Change-Id: I0dc1a106aa9e9531051c7d08867ace5ef230eb3f
Reviewed-on: https://go-review.googlesource.com/43310
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/mbitmap.go

@@ -364,6 +364,7 @@ func heapBitsForAddr(addr uintptr) heapBits {
 // heapBitsForSpan returns the heapBits for the span base address base.
 func heapBitsForSpan(base uintptr) (hbits heapBits) {
 	if base < mheap_.arena_start || base >= mheap_.arena_used {
+		print("runtime: base ", hex(base), " not in range [", hex(mheap_.arena_start), ",", hex(mheap_.arena_used), ")\n")
 		throw("heapBitsForSpan: base out of range")
 	}
 	return heapBitsForAddr(base)