Add support for fixed-point type comparison operators

This patch adds support for binary comparison operators with
fixed-point type values.

gdb/ChangeLog:

        * valarith.c (fixed_point_binop): Add BINOP_EQUAL and BINOP_LESS
        handling.
        (value_less): Add fixed-point handling.

gdb/testsuite/ChangeLog:

        * gdb.ada/fixed_cmp.exp: Add -fgnat-encodings=minimal testing.
        * gdb.dwarf2/dw2-fixed-point.c (pck__fp1_var2): New global.
        (main): Add reference to pck__fp1_var2.
        * gdb.dwarf2/dw2-fixed-point.exp: Add comparison operator testing.