s390x: snippets: Add snippet compilation

This patch provides the necessary make targets to properly compile
snippets and link them into their associated host.

To define the guest-host definition, we use the make-feature
`SECONDEXPANSION` in combination with `Target specific Variable
Values`. The variable `snippets` has different values depending on the
current target. This enables us to define which snippets (=guests)
belong to which hosts. Furthermore, using the second-expansion, we can
use `snippets` in the perquisites of the host's `%.elf` rule, which
otherwise would be not set.

Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
Acked-by: Thomas Huth <thuth@redhat.com>
Acked-by: Janosch Frank <frankja@linux.ibm.com>