They do not, but pretend that they do.
The immediate need is that it breaks the new GC because
these are weird symbols as if with pointers but not necessary
pointer aligned.

LGTM=rsc
R=golang-codereviews, dave, josharian, khr, rsc
CC=golang-codereviews, iant, khr, rlh
https://golang.org/cl/116060043