The tool was moved to tools/Testing/selftests within the Linux kernel
source tree. Adjust the URL in the comments of vdso_linux.go

Change-Id: I86b9cae4b898c4a45bc7c54891ce6ead91a22670
Reviewed-on: https://go-review.googlesource.com/87815Reviewed-by: Ian Lance Taylor <iant@golang.org>