x86 syscall.h: fix argument order

Petr Tesarik noticed that I'd bungled the syscall_get_arguments code for
64-bit kernels, so it inverted the order of the syscall argument registers.
Petr wrote a patch to fix that, and I've amended it to fix the same braino
in the syscall_set_arguments code.

Original-by: Petr Tesarik <ptesarik@suse.cz>
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>