I removed Tanuki's broken CPU "bitness" check for OSX in
3db46be1ede7d858b6c319905cf310d99227eb29. With this commit I add one that
works. Note that at this point we don't use the "bitness" checking because we
ship a quad-fat wrapper for OSX. Unfortunately, one can only manipulate the osx
binaries with "lipo" which is only available in OSX. In the future we may want
to use the "bitness" logic.

In any case, the old check set the bits to 64 for Leopard and above, but just
because a Mac is running Leopard or Snow Leopard doesn't mean it's 64bit
capable. This addition of mine will actually check the flags using sysctl. I'm
not using uname because OSX < Lion starts in 32bit mode by default (but can
still run 64bit binaries).