go.sys: fix imports to cgc/p/go.sys, add build tags

Approach but probably not achieve a green build.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/126120043

unix/mksyscall_solaris.pl

@@ -264,7 +264,7 @@ package $package
 import "unsafe"
 EOF
 
-print "import \"unix\"\n" if $package ne "unix";
+print "import \"code.google.com/p/go.sys/unix\"\n" if $package ne "unix";
 
 print <<EOF;