isl_val_gcdext: revert to open-coded version of isl_int_gcdext

isl_val_gcdext: revert to open-coded version of isl_int_gcdext

In 10a7f8a (isl_ast_build.c: use isl_int_gcdext instead of open-coded version,
Wed Apr 17 17:51:55 2013 +0200), the open-coded version was replaced
by a call to the isl_int_gcdext macro introduced in e191579
(add isl_int_gcdext, Wed Apr 17 17:51:28 2013 +0200).
In 0a83197 (try and avoid depending on mpz_gcdext internals,
Wed Jun 19 11:22:18 2013 +0200), we introduced a isl_gmp_gcdext
to try and mask differences in the outcomes of the underlying
mpz_gcdext over different versions of GMP.

However, there is no guarantee that we caught all the differences.
Furthermore, we will be introducing an alternative to GMP soon and
the differences across libraries are likely to be even greater than
those between different versions of the same library.

We therefore revert back to our (slightly modified) original implementation.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>