runtime: keep args and frame in struct Func

args is useful for printing tracebacks.

frame is not necessary anymore, but we might some day
get back to functions where the frame size does not vary
by program counter, and if so we'll need it. Avoid needing
to introduce a new struct format later by keeping it now.

Fixes #5907.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/13632051

src/pkg/runtime/runtime.h

@@ -433,7 +433,6 @@ struct	Func
 	uintptr	entry;	// start pc
 	int32	nameoff;// function name
 	
-	// TODO: Perhaps remove these fields.
 	int32	args;	// in/out args size
 	int32	frame;	// legacy frame size; use pcsp if possible