powerpc: Move code patching code into arch/powerpc/lib/code-patching.c

We currently have a few routines for patching code in asm/system.h, because
they didn't fit anywhere else. I'd like to clean them up a little and add
some more, so first move them into a dedicated C file - they don't need to
be inlined.

While we're moving the code, drop create_function_call(), it's intended
caller never got merged and will be replaced in future with something
different.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>