Handle 128-bit constants for fixed point

Handle 128-bit constants for fixed point

In some cases, GNAT can emit 128-bit constants for fixed-point types.
This patch changes gdb to handle this scenario, by changing the
low-level rational-reading functions in dwarf2/read.c to work directly
with gdb_mpz values.  (I'm not sure offhand if these 128-bit patches
have gone into upstream GCC yet -- but they will eventually, and
meanwhile I think it should be clear that this patch is otherwise
harmless.)

gdb/ChangeLog
2020-12-09  Tom Tromey  <tromey@adacore.com>

	* dwarf2/read.c (get_dwarf2_rational_constant): Change "numerator"
	and "denominator" to gdb_mpz.  Handle block forms.
	(get_dwarf2_unsigned_rational_constant): Change "numerator" and
	"denominator" to gdb_mpz.
	(finish_fixed_point_type): Update.
	(has_zero_over_zero_small_attribute): Update.