avr32: make architecture sys_clone prototype match asm-generic prototype

This patch will fix the arguments to the architecture sys_clone() function to
match the asm-generic/syscalls.h prototype. In the same go remove the
architecture specific prototype for the same function.

The sys_clone() function is only called from assembly, hence the argument types
were not having any affect.

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>