runtime: disable preemption

There are various problems, and both Dmitriy and I
will be away for the next week. Make the runtime a bit
more stable while we're gone.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/10848043

src/pkg/runtime/proc.c

@@ -2149,6 +2149,12 @@ preemptone(P *p)
 	M *mp;
 	G *gp;
 
+// Preemption requires more robust traceback routines.
+// For now, disable.
+// The if(1) silences a compiler warning about the rest of the
+// function being unreachable.
+if(1) return;
+
 	mp = p->m;
 	if(mp == nil || mp == m)
 		return;