Run fixed_points.exp with -fgnat-encodings=minimal

This changes the test case gdb.ada/fixed_points.exp to also be run
with -fgnat-encodings=minimal.  This change pointed out that the test
case had a few incorrect expected outputs; these are fixed as well.

Note that the Overprecise_Object test only uses the non-legacy output
with GCC trunk.

gdb/testsuite/ChangeLog
2020-12-18  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/fixed_points.exp: Also run with
	-fgnat-encodings=minimal.  Update expected output.