Only write gcov when file output is on [PR119553]

Only write gcov when file output is on [PR119553]

gcov_write_* functions must be guarded so they only are called when
output_to_file is true, like for -fcondition-coverage, otherwise it
triggers an invalid read as detected by valgrind. The gcno file is
mostly written to from profile.cc, so it doesn't make too much sense
to hide it in path-coverage.cc. The find_paths name was a bit
imprecise, and is renamed to instrument_prime_paths.

	PR gcov-profile/119553

gcc/ChangeLog:

	* path-coverage.cc (find_paths): Return path count, don't
	write to gcno, and rename to ...
	(instrument_prime_paths): ... this.
	* profile.cc (branch_prob): Write path counts to gcno.