Handle 64-bit system call arguments (off_t, id_t).

In particular, 64-bit system call arguments use up two register_t
arguments for 32-bit processes. They must also be aligned on a 64-bit
boundary on 32-bit powerpc processes. This fixes the decoding of
lseek(), procctl(), and wait6() arguments for 32-bit processes (both
native and via freebsd32).

Note that the ktrace system call return record only returns a single
register, so the return value of lseek is always truncated to the low
32-bits for 32-bit processes.