lib/s390x: Add gitignore file

asm-offsets.h and asm-offsets.s are generated files, they should
always be ignored by git.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>