rt: pull upstream ISAAC code for consistency between 32/64 bit platforms

The "unsigned 4 byte" `ub4`s are actually 8 bytes on 64-bit platforms
which mean that some bits > 2**32 were retained in calculations, these
would then "reappear" after a shift and so the stream of random numbers
would differ on 32 bit vs 64 bit platforms.

http://burtleburtle.net/bob/c/randport.c