cmd/compile: distinguish args and return values in DWARF

Set DW_AT_variable_parameter on DW_TAG_formal_parameters that are
actually return values. variable_parameter is supposed to indicate inout
parameters, but Go doesn't really have those, and DWARF doesn't have
explicit support for multiple return values. This seems to be the best
compromise, especially since the implementation of the two is very
similar -- both are stack slots.

Fixes #21100

Change-Id: Icebabc92b7b397e0aa00a7237478cce84ad1a670
Reviewed-on: https://go-review.googlesource.com/71670


Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>