ipc: prefer SYS_ipc when it is defined

ipc: prefer SYS_ipc when it is defined

Linux v5.1 introduced ipc syscalls on targets where previously only
SYS_ipc was available, change the logic such that the ipc code keeps
using SYS_ipc which works backward compatibly on older kernels.

This changes behaviour on microblaze which had both mechanisms, now
SYS_ipc will be used instead of separate syscalls.