runtime: fix traceback of trap on ARM

The merged traceback was wrong for LR machines,
because traceback didn't pass lr to gentraceback.
Now that we have a test looking at traceback output
for a trap (the test of runtime.Breakpoint),
we caught this.

While we're here, fix a 'set and not used' warning.

Fixes arm build.

TBR=r
R=r
CC=golang-codereviews
https://golang.org/cl/143040043

src/runtime/mgc0.c

@@ -634,9 +634,10 @@ scanframe(Stkframe *frame, void *unused)
 
 	// Scan local variables if stack frame has been allocated.
 	size = frame->varp - frame->sp;
-	minsize = 0;
 	if(thechar != '6' && thechar != '8')
 		minsize = sizeof(uintptr);
+	else
+		minsize = 0;
 	if(size > minsize) {
 		stackmap = runtime·funcdata(f, FUNCDATA_LocalsPointerMaps);
 		if(stackmap == nil || stackmap->n <= 0) {

src/runtime/traceback.go

@@ -451,9 +451,9 @@ func traceback(pc uintptr, sp uintptr, lr uintptr, gp *g) {
 	}
 	// Print traceback. By default, omits runtime frames.
 	// If that means we print nothing at all, repeat forcing all frames printed.
-	n = gentraceback(pc, sp, 0, gp, 0, nil, _TracebackMaxFrames, nil, nil, false)
+	n = gentraceback(pc, sp, lr, gp, 0, nil, _TracebackMaxFrames, nil, nil, false)
 	if n == 0 {
-		n = gentraceback(pc, sp, 0, gp, 0, nil, _TracebackMaxFrames, nil, nil, true)
+		n = gentraceback(pc, sp, lr, gp, 0, nil, _TracebackMaxFrames, nil, nil, true)
 	}
 	if n == _TracebackMaxFrames {
 		print("...additional frames elided...\n")