I-pipe makes sure that arguments of compat calls are ordered just like
native calls. Dovetail does not. But it is better to use the kernel's
syscall_get_arguments for retrieving the arguments anyway. Introduce
pipeline_get_syscall_args to abstract that difference.

Xenomai/ARM syscalls have an unconventional argument marshalling, with
ARM_ORIG_r0 containing the Xenomai syscall number (ORed with
__COBALT_SYSCALL_BIT), followed by the first actual argument to the
syscall in ARM_r1.

For this reason, using syscall_get_arguments() to collect the syscall
parameters won't yield the expected result, with regs[0] matching
__xn_reg_sys(regs) instead of __xn_reg_arg1(regs).

Allow Dovetail-enabled architectures to override the generic
implementation of pipeline_get_syscall_args(), provide a working one
for ARM.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
[Philippe: add special handling of ARM]
Signed-off-by: Philippe Gerum <rpm@xenomai.org>