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: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -4938,6 +4938,7 @@ echo "QEMU_CFLAGS=$QEMU_CFLAGS" >> $config_host_mak
 echo "QEMU_INCLUDES=$QEMU_INCLUDES" >> $config_host_mak
 if test "$sparse" = "yes" ; then
   echo "CC           := REAL_CC=\"\$(CC)\" cgcc"       >> $config_host_mak
+  echo "CPP          := REAL_CC=\"\$(CPP)\" cgcc"      >> $config_host_mak
   echo "CXX          := REAL_CC=\"\$(CXX)\" cgcc"      >> $config_host_mak
   echo "HOST_CC      := REAL_CC=\"\$(HOST_CC)\" cgcc"  >> $config_host_mak
   echo "QEMU_CFLAGS  += -Wbitwise -Wno-transparent-union -Wno-old-initializer -Wno-non-pointer-null" >> $config_host_mak