rt: Rework lio_listio implementation

Move the common code into rt/lio_listio-common.c and include
the file in both rt/lio_listio.c and rt/lio_listio64.c.  The common
code automatically defines both public symbols for __WORDSIZE == 64.

Reviewed-by: Adhemerva Zanella <adhemerval.zanella@linaro.org>