BUG/MINOR: syscall: fix NR_accept4 system call on sparc/linux

An invalid copy-paste called it NR_splice instead of NR_accept4.
This does not lead to real issues because if this define is used,
then the code cannot compile since NR_accept4 is still missing.
BUILD/MINOR: syscall: add definition of NR_accept4 for ARM

This platform was not covered and older libc do not provide accept4().