Replace the confusing game where a frame size of $-8 would suppress the
implicit setting up of a stack frame with a nice explicit flag.

The code to set up the function prologue is still a little confusing but better
than it was.

Change-Id: I1d49278ff42c6bc734ebfb079998b32bc53f8d9a
Reviewed-on: https://go-review.googlesource.com/15670Reviewed-by: Minux Ma <minux@golang.org>