Revision 01810e5c68e9 added the following to src/pkg/runtime/stack.h:

        StackPreempt = (uintptr)(intptr)0xffffade,

The typedef for intptr is defined in two places:

        1. src/pkg/runtime/runtime.h for use by the runtime

        2. include/u.h for use by the compilers and linkers

On Plan 9, we don't use include/u.h but instead augment the host's
u.h with extra typedefs. These are in include/plan9/GOARCH/u.h.

We forgot to add intptr. It didn't cause a problem until now since
that typedef was never used outside the runtime.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/10023043