O32 programs put arguments, that can't be put on argument registers
(a[0-3] for O32), on stack, where 64-bit values are aligned to 64-bit
and 32-bit and smaller values are aligned to 32-bit.

The new syscall entry code Matt Thomas has just added (thanks!) to
matt-mips64 doesn't work if register_t is 64-bit.  The right code I
think would be

        * copyin() the whole arguments (sy_argsize) into kernel, and
        * copy each argument into register_t array.

But but we can't know sizes of each argument at this point.  Some may
be 64-bit, like off_t.  So we can't rearrange arguments into
register_t array.

Masao