Have `symlink_preferred_arch: true` honor the `stem` property.

If a module defines a `stem` property, use it to name the symlink
created for `symlink_preferred_arch: true` (instead of the module's
name).

Also always require suffixes when using `symlink_preferred_arch: true`.

Test: m
Change-Id: Ia8a56f94d21599194797ef8b259f5f03e08cb16a