malloc checks kindNoPointers and if it is not set and the object
is one pointer in size, it assumes it contains a pointer.  So we
must set kindNoPointers correctly; it isn't just a hint.

Fixes #9425

Change-Id: Ia43da23cc3298d6e3d6dbdf66d32e9678f0aedcf
Reviewed-on: https://go-review.googlesource.com/2055Reviewed-by: Russ Cox <rsc@golang.org>