avoid unwarranted assumption in gdb.ada/fixed_points/fixed_points.adb

avoid unwarranted assumption in gdb.ada/fixed_points/fixed_points.adb

The test program being used declares a fixed-point type
(Base_Fixed_Point_Type) Base_Fixed_Point_Type whose (scaled) range
is System.Min_Int .. System.Max_Int. is an unwarranted assumption because
the range is implementation-defined. It means the compiler is therefore
free to reject that declaration.

We noticed this while one of my coworkers was working on enhancing
GNAT to support 128bit integers. The bulk of the work has been done,
but one side-effect is that there is a small gap in this particular
area where the compiler is now rejecting this code. We will eventually
plug that gap, but in meantime, since the testcase itself doesn't really
need such a large range, this commit simply adjusts the test program
to use hard-coded bounds for the range whose value are more reasonable.

gdb/testsuite/ChangeLog:

        * gdb.ada/fixed_points/fixed_points.adb: Replace use of
        System.Min_Int and System.Max_Int with smaller hardcoded
        constants.