Fix printing of Wide_Character & Wide_Wide_Character entities.

Fix printing of Wide_Character & Wide_Wide_Character entities.

Wide_Characters and Wide_Wide_Characters are incorrectly printed.
Consider for instance:

    Medium : Wide_Character := Wide_Character'Val(16#dead#);

Trying to print the value of this variable yields:

    (gdb) p medium
    $1 = 57005 '["ad"]'

The integer value is correct (57005 = 0xdead), but the character
representation is not, it should be:

    $1 = 57005 '["dead"]'

Same for Wide_Wide_Characters.

There were two issues:
   (a) The first issue was in ada-valprint, where we were assuming
       that character types were 1 byte long;
   (b) The second problem was in c-valprint, where we were down-casting
       the integer value of the character to type `unsigned char',
       causing use to lose all but the lowest byte.

gdb/ChangeLog:

        * ada-valprint. (ada_printchar): Use the correct type length
        in call to ada_emit_char.
        * c-valprint.c (c_val_print): Remove cast in call to LA_PRINT_CHAR.