powerpc: Wire up direct socket system calls

This patch wires up the various socket system calls on PowerPC so that
userspace can call them directly, rather than by going through the
multiplexed socketcall system call.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>