runtime: do not handle signals before configuring handler

There was a small window during program initialization
where a signal could come in before the handling mechanisms
were set up to handle it.  Delay the signal-handler installation
until we're ready for the signals.

Fixes #3314.

R=golang-dev, dsymonds, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5833049

src/pkg/runtime/proc.c

@@ -734,6 +734,12 @@ runtime·mstart(void)
 	m->g0->sched.pc = (void*)-1;  // make sure it is never used
 	runtime·asminit();
 	runtime·minit();
+
+	// Install signal handlers; after minit so that minit can
+	// prepare the thread to be able to handle the signals.
+	if(m == &runtime·m0)
+		runtime·initsig();
+
 	schedule(nil);
 }
 

src/pkg/runtime/runtime.c

@@ -342,8 +342,6 @@ runtime·check(void)
 		runtime·throw("float32nan2");
 	if(!(i != i1))
 		runtime·throw("float32nan3");
-
-	runtime·initsig();
 }
 
 void