[MIPS] Fix some system calls with long long arguments

* O32 fadvise64() pass long long arguments by register pairs.  Add
  sys32 version for 64 bit kernel.
* N32 readahead() can pass a long long argument by one register.  No
  need to use sys32_readahead.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>