[POWERPC] Mask 32-bit system call arguments to 32 bits on PPC64 in audit code

The system call entry code will clear the high bits of argument
registers before invoking the system call; don't report whatever noise
happens to be in the high bits of the register before that happens.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>