Needed for invoking a Go subprocess in the C code.
The Go tools live in $GOROOT/pkg/tool/$GOHOSTARCH_$GOHOSTOS.

Change-Id: I961b6b8a07de912de174b758b2fb87d77080546d
Reviewed-on: https://go-review.googlesource.com/3042Reviewed-by: Ian Lance Taylor <iant@golang.org>