mips64: add missing system call numbers

r14384 introduced use of getdents64 syscall and we missed a system call
value for MIPS64, so it broke the build for it. Add missing values now.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14409 a5019735-40e9-0310-863c-91ae7b9d1cf9