uclibc: add prctl syscall fix for AVR32 architecture

This patch fixes the prctl syscall for the AVR32 architecture, since this
architecture assumes you handle varargs in a proper way, always.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>