s390/kexec_file: Add kexec_file_load system call

This patch adds the kexec_file_load system call to s390 as well as the arch
specific functions common code requires to work. Loaders for the different
file types will be added later.

Signed-off-by: Philipp Rudo <prudo@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>