mips n32: fix syscall table

mips n32: fix syscall table

* linux/mips/syscallent-n32.h: Add entries for mbind, get_mempolicy,
set_mempolicy and timerfd syscalls.  Fix entry for socket syscall.
Fix handlers for truncate, ftruncate and getdents64 syscalls.
Fix names for pread64, pwrite64, shmat, rt_sigsuspend, sigaltstack,
umount2, and getdents64 syscalls.