Detecting 64-bit Platforms

[Mike Barnes <strepsil gmail com>]

In reference to this:
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=122304

Just wondered if anyone had a good way to detect a 64-bit platform in
the context of an RPM spec file. While building for the Alpha, we've
come across similar situations a couple of times, where we turn up a
bug that's already patched, but it's %ifarch-ed to x86_64 or whatever
or, like in this case, just checking for "lib64" being used instead of
"lib".

Aside from just manually updating ifarch lists for patches, is there
any good way to just pull in a patch on any system that happens to be
64-bit? Of course, if there's not, that's no big deal, but thought it
might be worth mentioning in case someone comes up with something
brilliant. :)