runtime: disable Native Client fake time support

This CL is a reformulation of CL 73110043 containing only the minimum required to get the nacl builds compiling.

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/74220043

src/pkg/runtime/time.goc

@@ -21,6 +21,9 @@ static Timers timers;
 static void addtimer(Timer*);
 static void dumptimers(int8*);
 
+// nacl fake time support. 
+int64 runtime·timens;
+
 // Package time APIs.
 // Godoc uses the comments in package time, not these.