64bit variant naming scheme

[Bryan Blackburn]

Currently there are only a few ports with 64bit variants:

 nbench-byte +use_64_bit
 ubench +use_64_bit
 judy +bit64
 john-devel +use_64_bit

(john-devel was updated with 64bit support by me, picking the more common
name).

Before there are many more ports with such variants, we need to decide on a
standard name; I found that +64bit isn't liked by port as it thinks that is
the name of a port, not a variant so I guess it doesn't like a variant name
to start with a number.

The only issue I have with +use_64_bit is it's long and a pain to type...

Bryan