[Ada] Fix printing of Wide_Wide_Strings

[Ada] Fix printing of Wide_Wide_Strings

This fixes the printing of Wide_Wide_String objects.  For instance,
consider:

    My_WWS : Wide_Wide_String := " helo";

Before this patch is applied, GDB prints:

    (gdb) print my_wws
    $1 = " ["00"]h["00"]e"

gdb/ChangeLog:

        * ada-valprint.c (ada_emit_char): Remove strange code.
        Check that c is <= UCHAR_MAX before passing it to isascii.
        (char_at): Do not assume that TYPE_LEN is either 1 or 2.