s390x: add kdump sysconfig option to use the kexec_file_load() syscall
    
    Currently, kernel has supported the kexec_file_load() syscall on s390x,
    so I follow this commit:
    6a20bd54473e ("kdump-lib: switch to the kexec_file_load() syscall on
    x86_64 by default")
    
    And also enable the kexec_file_load() syscall for kdump on s390x by
    default.
    
    Signed-off-by: Lianbo Jiang <lijiang@redhat.com>
    Acked-by: Kairui Song <kasong@redhat.com>