Require a name to be specified when referencing the pseudo-stack.
If you want a real stack offset, use the hardware stack pointer (e.g.,
R13 on arm), not SP.

Fix affected assembly files.

Change-Id: If3545f187a43cdda4acc892000038ec25901132a
Reviewed-on: https://go-review.googlesource.com/5120
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>