PPC32: Add a thread-pointer argument to the clone syscall, make a prepare_to_copy().

PPC32: Add a thread-pointer argument to the clone syscall, make a prepare_to_copy().

The thread-pointer argument gets copied to R2 in the child in copy_thread() if
the CLONE_SETTLS flag is set.  Adding a prepare_to_copy simplifies the copy_thread
logic since we don't have to do the extra copy of fpu/altivec state to the child.