linux(4): Call semop directly.

As the Linux semop syscall is not defined in i386, and as it is equal
to the native semop syscall, call it directly.
Fix semop definition to match Linux actual one - nsops is size_t type.

MFC after: 2 weeks

(cherry picked from commit f686092664ee6c2c287f77f716db4d72e451c780)