Update gcc-internal-format for GCC 15.

Update gcc-internal-format for GCC 15.

Reported by Roland Illig, Andrew Pinski, Jakub Jelinek in
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119684>.
Also reported by Roland Illig in
<https://savannah.gnu.org/bugs/?50450>, <https://savannah.gnu.org/bugs/?50461>.

* gettext-tools/src/format-gcc-internal.c: Update for GCC 15.
* gettext-tools/tests/format-gcc-internal-1: Likewise.
* gettext-tools/tests/format-gcc-internal-2: Likewise.
* gettext-tools/doc/gettext.texi (gcc-internal-format): Likewise.