linux-user: create target_structs header to place ipc_perm and shmid_ds

linux-user: create target_structs header to place ipc_perm and shmid_ds

Creating target_structs header in linux-user/$arch/ and making
target_ipc_perm and target_shmid_ds its first inhabitants.
The struct defintions may/should be further fine-tuned by arch maintainers.

Signed-off-by: Petar Jovanovic <petar.jovanovic@imgtec.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
(cherry picked from commit 55a2b1631fb343edac4a2d4596c72e58ee1372b3)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>