userspace: minor typo fixes in various places

System call arguments are indexed from 1 to 6, so arg0
is corrected to arg1 in two occasions. In addition, the
ARM function for system calls is now called z_arm_do_syscall,
so we update the inline comment in __svc handler.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>