Fix uatomic sign cast

Passing an unsigned int to uatomic_sub does not honor sign extend to
long, as we should be allowed by assume.

Fix this by introducing caa_cast_long_keep_sign(), which casts either to
long or unsigned long depending on the signedness of the argument
received. It is used in uatomic_sub before applying the "-" operator,
since this operator needs to operate on the "long" type size (since sign
extension might not be performed if the argument received is unsigned).

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>