kbuild: fix a bug of C++ host program handling

The comment claims:
  C++ executables compiled from at least one .cc file
  and zero or more .c files

But C++ executables with zero .c file fail in build.

For example, assume we have a Makefile like this:

  hostprogs-y := foo
  foo-cxxobjs := bar.o

In this case, foo is treated as host-csingle
and Kbuild tries to search non-existing foo.c source.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>