intl: Annotate *printf functions with __attribute__ __format__.

intl: Annotate *printf functions with __attribute__ __format__.

Reported by Pierre Ossman <ossman@cendio.se> at
<https://savannah.gnu.org/bugs/?64384>.

* gettext-runtime/intl/libgnuintl.in.h (_INTL_ATTRIBUTE_FORMAT,
_INTL_ATTRIBUTE_SPEC_PRINTF_STANDARD, _INTL_ATTRIBUTE_FORMAT_PRINTF_STANDARD):
New macros.
(fprintf, vfprintf, printf, vprintf, sprintf, vsprintf, snprintf, vsnprintf,
asprintf, vasprintf): Annotate with _INTL_ATTRIBUTE_FORMAT_PRINTF_STANDARD.
* gettext-runtime/intl/printf.c: Update comment.