Fix 'prctl' system call for IRIX. At this point IRIX 5.3 static binaries

are now working for 80% of the ones I have tried. The other ones that do
not work all fail in the same way with the same messages. Once that bug
is tracked down, we should be in good shape. Task locking still needs
some work.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>