mips: enable syscalls prlimit64, process_vm_readv, process_vm_writev

Enable wrappers for syscalls prlimit64, process_vm_readv, process_vm_writev,
needed by the following tests:

- none/tests/rlimit64_nofile and
- none/tests/process_vm_readv_writev.

The change also adds definitions for several system calls for MIPS64.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13400 a5019735-40e9-0310-863c-91ae7b9d1cf9