ia64: fix set-but-unused warnings with syscalls

ia64: fix set-but-unused warnings with syscalls

These macros often set up a variable that later macros sometimes do
not use.  Add unused attribute to avoid that.

Similarly, the ia64 code tends to check the err field rather than
the val (which is opposite of most arches) leading to the same
kind of warning.  Replace this with a dummy reference.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>