Use C library functions instead of Abs() for int64

Use C library functions instead of Abs() for int64

Instead of Abs() for int64, use the C standard functions labs() or
llabs() as appropriate.  Define a small wrapper around them that
matches our definition of int64.  (labs() is C90, llabs() is C99.)

Reviewed-by: Zhang Mingli <zmlpostgres@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/4beb42b5-216b-bce8-d452-d924d5794c63%40enterprisedb.com