sparse: Fix build with sparse on .S files rules.mak has a rule for .S files using CPP. This will result in errors like CPP s390-ccw/start.asm cc: error: unrecognized command line option '-Wbitwise' Lets also redefine CPP in case of --enable-sparse. Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>