Remove ia64*-*-linux from the list of obsolete targets

The following un-deprecates ia64*-*-linux for GCC 15. Since we plan to
support this for some years to come.

gcc/
	* config.gcc: Only list ia64*-*-(hpux|vms|elf) in the list of
	obsoleted targets.

contrib/
	* config-list.mk (LIST): no --enable-obsolete for ia64-linux.

Signed-off-by: René Rebe <rene@exactcode.de>