arch/ia64/lib: Fix broken URL in comments

The URL to the book IA-64 and Elementary Functions in idiv32.S and
idiv64.S just led to a 404 page, so I updated them with a known good link
that others can reference.

Signed-off-by: Sina Hamedian <shamedian@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>