fix printing of DWARF fixed-point type objects with format modifier

Consider a fixed-point type such the scaling factor is 1/16,
as the following Ada code snippet would create:

      type FP1_Type is delta 0.1 range -1.0 .. +1.0;
      FP1_Var : FP1_Type := 0.25;

Printing the value of this variable with a format modifier yields
the wrong value. E.g.:

    (gdb) p /x fp1_var
    $6 = 0x4

Since the real value is 0.25, we therefore expected...

    (gdb) p /x fp1_var
    $6 = 0x0

What happens, in this case, is that the value being printed is
actually the "raw" value of our object, before the scaling factor
gets applied.

This commit fixes the issue by using approach as for float values,
where we convert the value into an integer value, prior to printing,
knowing that the conversion takes the scaling factor into account.

gdb/ChangeLog:

        * printcmd.c (print_scalar_formatted): Add fixed-point type
        handling when options->format is set.

gdb/testsuite/ChangeLog:

        * gdb.dwarf2/dw2-fixed-point.exp: Add "print /x" tests.