Issues with > 3G of ram fixed for 64 bit builds

[elekktretterr at exemail.com.au]

>     x86_64 now works properly with > 3G of ram.  Bounce buffers in the
> kernel
>     were broken due to an incorrect #define.

I guess this could be why ciss on x86_64 (machine also has 5GB RAM) halts
the boot process.

Petr