runtime: update URL of the Linux vDSO parser tool

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>

src/runtime/vdso_linux.go

@@ -12,7 +12,7 @@ import "unsafe"
 // Look up symbols in the Linux vDSO.
 
 // This code was originally based on the sample Linux vDSO parser at
-// https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/vDSO/parse_vdso.c
+// https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/vDSO/parse_vdso.c
 
 // This implements the ELF dynamic linking spec at
 // http://sco.com/developers/gabi/latest/ch5.dynamic.html