Fix fixed-point binary operation type handling

Testing showed that gdb was not correctly handling some fixed-point
binary operations correctly.

Addition and subtraction worked by casting the result to the type of
left hand operand.  So, "fixed+int" had a different type -- and
different value -- from "int+fixed".

Furthermore, for multiplication and division, it does not make sense
to first cast both sides to the fixed-point type.  For example, this
can prevent "f * 1" from yielding "f", if 1 is not in the domain of
"f".  Instead, this patch changes gdb to use the value.  (This is
somewhat different from Ada semantics, as those can yield a "universal
fixed point".)

This includes a new test case.  It is only run in "minimal" mode, as
the old-style fixed point works differently, and is obsolete, so I
have no plans to change it.

gdb/ChangeLog
2021-01-06  Tom Tromey  <tromey@adacore.com>

	* ada-lang.c (ada_evaluate_subexp) <BINOP_ADD, BINOP_SUB>:
	Do not cast result.
	* valarith.c (fixed_point_binop): Handle multiplication
	and division specially.
	* valops.c (value_to_gdb_mpq): New function.
	(value_cast_to_fixed_point): Use it.

gdb/testsuite/ChangeLog
2021-01-06  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/fixed_points/pck.ads (Delta4): New constant.
	(FP4_Type): New type.
	(FP4_Var): New variable.
	* gdb.ada/fixed_points/fixed_points.adb: Update.
	* gdb.ada/fixed_points.exp: Add tests for binary operators.