powerpc: Turn syscall handler into macros

This turns the syscall handler into macros as we are going to want to reuse
them again later.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>