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>