runtime: disable preemption again to fix linux build

Otherwise the tests in pkg/runtime fail:

runtime: unknown argument frame size for runtime.deferreturn called from 0x48657b [runtime_test.func·022]
fatal error: invalid stack
...

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/11483043

src/pkg/runtime/proc.c

@@ -2175,6 +2175,12 @@ 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;