s390x: snippets: Define all things that are needed to link the libc

In the long run, we want to use parts of the libc like memset() etc.,
too. However, to be able to link it correctly, we have to provide
some stub functions like puts() and exit() to avoid that too much
other stuff from the lib folder gets pulled into the binaries, which
we cannot provide in the snippets (like the sclp support).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211008092649.959956-1-thuth@redhat.com>
Link: https://lore.kernel.org/kvm/20211008092649.959956-1-thuth@redhat.com/


Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>