avr32: wire up preadv2 and pwritev2 syscalls

This patch wires up the new preadv2 and pwritev2 syscall on AVR32.

On AVR32, all parameters beyond the 5th are passed on the stack. System
calls don't use the stack -- they borrow a callee-saved register
instead. This means that syscalls that take 6 parameters must be called
through a stub that pushes the last parameter on the stack.

Signed-off-by: Hans-Christian Noren Egtvedt <egtvedt@samfundet.no>