cobalt/syscall: pipeline: abstract syscall entry points

The I-pipe and Dovetail differ only marginally with respect to syscall
handling. Abstract only the few details we need to cope with both
interfaces.

No functional change is introduced.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
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