On the plus side, we don't need to change the bits when mallocing
pointerless objects.  On the other hand, we need to mark objects in the
free lists during GC.  But the free lists are small at GC time, so it
should be a net win.

benchmark                    old ns/op    new ns/op    delta
BenchmarkMalloc8                    40           33  -17.65%
BenchmarkMalloc16                   45           38  -15.72%
BenchmarkMallocTypeInfo8            58           59   +0.85%
BenchmarkMallocTypeInfo16           63           64   +1.10%

R=golang-dev, rsc, dvyukov
CC=cshapiro, golang-dev
https://golang.org/cl/41040043