bitwise: do not remove the signedness of bitwise types

When bitwise types were added [1] the signedness modifiers
were removed from them. More exactly, it was MOD_SPECIFIER
which was removed. I suppose this was done because then
MOD_SPECIFIER contained the signedness bits but also MOD_CHAR,
MOD_LONG, ...  and those had to be removed.

But currently MOD_SPECIFIER contains only MOD_SIGNEDNESS
and the signedness info can be useful for bitwise types too.

So, do not removed anymore MOD_SPECIFIER from the bitwise
types' modifiers.

[1] commit 032f492af0ac ("[PATCH] __attribute__((bitwise))")

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>