Previously, I had made available a tarball of
the modified system headers that were necessary
to build on Plan 9 but that was only a stopgap.
I think this method is much better since no
files outside of $GOROOT will have to be added
or modified during the build process.

Also, this is just the first step. I'll change
the build to reference these files in another CL
(that also contains a few more Makefile changes).

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5552056