runtime: zero pointer-looking scalararg values

I have a CL which at every gc looks through data and bss
sections for nonpointer data (according to gc maps) that
looks like a pointer.  These are potential missing roots.

The only thing it finds are begnign, storing stack pointers
into m0.scalararg[1] and never cleaning them up.  Let's
clean them up now so the test CL passes all.bash cleanly.

The test CL can't be checked in because we might store
pointer-looking things in nonpointer data by accident.

LGTM=iant
R=golang-codereviews, iant, khr
CC=golang-codereviews
https://golang.org/cl/153210043

src/runtime/panic.c

@@ -31,6 +31,7 @@ runtime·deferproc_m(void)
 	argp = g->m->scalararg[1];
 	callerpc = g->m->scalararg[2];
 	g->m->ptrarg[0] = nil;
+	g->m->scalararg[1] = 0;
 
 	d = runtime·newdefer(siz);
 	d->fn = fn;
@@ -131,6 +132,7 @@ runtime·dopanic_m(void)
 	g->m->ptrarg[0] = nil;
 	pc = g->m->scalararg[0];
 	sp = g->m->scalararg[1];
+	g->m->scalararg[1] = 0;
 	if(gp->sig != 0)
 		runtime·printf("[signal %x code=%p addr=%p pc=%p]\n",
 			gp->sig, gp->sigcode0, gp->sigcode1, gp->sigpc);