Common list of 64bit platforms

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
I would like to add a common variable to enumerate the 64bit platforms.
There are a number of packages that are practically impossible to fix
for 64bit platforms and it would be nice to mark them as unsupported
without having to repeat the list over and over again.

Can I add
        LP64PLATFORMS=  *-*-alpha *-*-sparc64 *-*-x86_64
to bsd.prefs.mk and start using it by adding e.g.
        NOT_FOR_PLATFORMS=      ${LP64PLATFORMS}
to emulators/arcem, just to name one culprit.

Joerg