Switch 64-bit sparcv9 platforms from bn(64,64) to bn(64,32). This doesn't

have impact on performance, because amount of multiplications does not
increase with this switch, not on sparcv9 that is. On the contrary, it
actually improves performance, because it spares a load of instructions
used to chase carries. Not to mention that BN assembler modules can be
shared more freely between 32- and 64-bit builts.