lib: s390x: snippet.h: Add a few constants that will make our life easier

The variable names for the snippet objects are of gigantic length so
let's define a few macros to make them easier to read.

Also add a standard PSW which should be used to start the snippet.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>