gcov-tool: Allow merging of empty profile lists

The gcov_profile_merge() already had code to deal with profile information
which had no counterpart to merge with.  For profile information from files
with no associated counterpart, the profile information is simply used as is
with the weighting transformation applied.  Make sure that gcov_profile_merge()
works with an empty target profile list.  Return the merged profile list.

gcc/
	* gcov-tool.cc (gcov_profile_merge): Adjust return type.
	(profile_merge): Allow merging of directories which contain no profile
	files.

libgcc/
	* libgcov-util.c (gcov_profile_merge): Return the list of merged
	profiles.  Accept empty target and source profile lists.