pgsql: 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(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://www.postgresql.org/message-id/flat/4beb42b5-216b-bce8-d452-d924d5794c63%40enterprisedb.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/357cfefb09115292cfb98d504199e6df8201c957

Modified Files
--------------
contrib/btree_gist/btree_cash.c | 2 +-
contrib/btree_gist/btree_int8.c | 2 +-
src/backend/utils/adt/datetime.c | 8 ++++----
src/backend/utils/adt/dbsize.c | 2 +-
src/include/c.h | 9 +++++++++
5 files changed, 16 insertions(+), 7 deletions(-)