Make gen-libm-test.py treat plus_oflow and minus_oflow as non-finite.

When converting gen-libm-test to Python, in one place I noted a bug in
the old Perl version that I preserved in the Python version so that
the generated output files were the same with both versions, as such
comparisons help give confidence in the correctness of such a rewrite
of a script.  Now that the conversion has been done, this patch fixes
that bug, by arranging for tests with plus_oflow or minus_oflow
results (manually written tests in libm-test-*.inc that have
overflowing results that thus depend on the rounding mode) to be
properly treated as having non-finite results, and thus not run for
the __FINITE_MATH_ONLY__ tests.  (As the affected tests in fact did
pass for __FINITE_MATH_ONLY__ testing, this is just a matter of
logical correctness in the choice of which tests run for that case,
rather than fixing any actual test failures.)

Tested for x86_64.

	* math/gen-libm-test.py (gen_test_args_res): Also treat plus_oflow
	and minus_oflow as non-finite.