Make accept4 for MIPS o32 avoid socketcall.

Make accept4 for MIPS o32 avoid socketcall.

sysdeps/unix/sysv/linux/internal_accept4.S expects socket.S to be
present if __NR_socketcall is defined (which it is on MIPS o32, even
though there are separate syscalls as well) and __NR_accept4 isn't.
MIPS does not have socket.S, since it uses separate syscalls, but
though the accept4 syscall should be added soon present kernel headers
do not have it.  This patch creates a dummy internal_accept4.S for
MIPS o32, and an accept4.c wrapper that undefines __NR_socketcall so
that the main accept4.c falls back to the ENOSYS implementation if
__NR_accept4 isn't defined; it doesn't seem worthwhile to have a
special socketcall-based assembly implementation just for o32 on a few
kernels in the range between accept4 being available via socketcall
and the accept4 syscall being available.