"-8" is not a sensible frame size on arm and we're about to start
rejecting it. Replace it with -4.

Likewise, "-4" is not a sensible frame size on arm64 and we're about
to start rejecting it. Replace it with -8.

Finally, clean up some places we're weirdly inconsistent about using 0
versus -8.

Change-Id: If85e229993d5f7f1f0cfa9852b4e294d053bd784
Reviewed-on: https://go-review.googlesource.com/92038
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>