mips32: fix fadvise64 system call

mips32: fix fadvise64 system call

For fadvise64 system call, 7th 32-bit argument slot (third on the stack)
will also be used due to MIPS O32 calling convention in passing 64-bit
values.

sys_fadvise64(int fd, loff_t offset, loff_t len, int advice);

NR_fadvise64 -> v0               (sysno)
fd           -> a0               (ARG1)
offset       -> a2, a3           (ARG3, ARG4)
len          -> SP + 16, SP + 20 (ARG5, ARG6)
advise       -> SP + 24          (ARG7)

Change the code according to it.

Patch by Aleksandar Rikalo.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16162