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 issue 5907.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13632051