doc/compat1.html: link to go.sys

You talked me into it. This and other links should be updated
once the new import paths for the subrepos are established.

LGTM=minux
R=golang-codereviews, minux
CC=golang-codereviews
https://golang.org/cl/124260043

doc/go1compat.html

@@ -161,7 +161,8 @@ The <a href="/pkg/syscall/"><code>syscall</code></a> package
 is therefore outside the purview of the guarantees made here.
 As of Go version 1.4, the <code>syscall</code> package is frozen.
 Any evolution of the system call interface must be supported elsewhere,
-such as in the <code>go.sys</code> subrepository.
+such as in the
+<a href="http://godoc.org/code.google.com/p/go.sys">go.sys</a> subrepository.
 For details and background, see
 <a href="https://golang.org/s/go1.4-syscall">this document</a>.
 </p>